Now his father, King David, had never disciplined him at any time, even by asking, “Why are you doing that?” Adonijah had been born next after Absalom, and he was very handsome. (1 Kings 1:6 NLT)

This verse is given to us so that we can clearly see the link between the absence of discipline and the outcome of folly and self-destruction, which is what happened to Adonijah. 

There are some of you that do not believe in corporal punishment (i.e. spanking), and even others that do not believe in any kind of discipline for your children. Instead you treat your children like you would adults and try to reason with them. This is pure folly. Children are simple-minded with a penchant for folly. The folly that the Bible says is bound up in your child (Proverbs 22:15) has also infected you. When you refuse to discipline your child, it is like disowning your child, and allowing other forces to either discipline them or destroy them. This is certainly not wise parenting. Consider the following passages from the wisdom of the Proverbs:

Those who spare the rod of discipline hate their children. Those who love their children care enough to discipline them. (Proverbs 13:24 NLT)

Discipline your children while there is hope. Otherwise you will ruin their lives. (Proverbs 19:18 NLT)

A youngster’s heart is filled with foolishness, but physical discipline will drive it far away. (Proverbs 22:15 NLT)

Don’t fail to discipline your children. They won’t die if you spank them. Physical discipline may well save them from death. (Proverbs 23:13, 14 NLT)

To discipline a child produces wisdom, but a mother is disgraced by an undisciplined child. (Proverbs 29:15 NLT)

Discipline your children, and they will give you peace of mind and will make your heart glad. (Proverbs 29:17 NLT)

What is notable in Proverbs is that the counsel to discipline children is more prevalent than any other principle for parents with respect to their children. If you err in the application of discipline, it is better to err in giving too much discipline than in giving too little discipline. The necessity of discipline is echoed in the New Testament:

Fathers, do not provoke your children to anger by the way you treat them. Rather, bring them up with the discipline and instruction that comes from the Lord. (Ephesians 6:4 NLT)

I too am a parent, even a father, and so I advise you parents, and particularly you fathers, do not fail in this responsibility. Just as you are responsible to love your wives with the love of Christ and to wash them with the Word of God, you are also responsible for bringing the Lord's instruction and discipline to your children. Let us do this with faithfulness, wisdom and diligence. 

1 Kings  2:15-22 puzzled me.

⁵ “As you know,” he said, “the kingdom was mine. All Israel looked to me as their king. But things changed, and the kingdom has gone to my brother; for it has come to him from the LORD. ¹⁶ Now I have one request to make of you. Do not refuse me.”

   “You may make it,” she said.

 ¹⁷ So he continued, “Please ask King Solomon—he will not refuse you—to give me Abishag the Shunammite as my wife.”

 ¹⁸ “Very well,” Bathsheba replied, “I will speak to the king for you.”

 ¹⁹ When Bathsheba went to King Solomon to speak to him for Adonijah, the king stood up to meet her, bowed down to her and sat down on his throne. He had a throne brought for the king’s mother, and she sat down at his right hand.

 ²⁰ “I have one small request to make of you,” she said. “Do not refuse me.”

   The king replied, “Make it, my mother; I will not refuse you.”

 ²¹ So she said, “Let Abishag the Shunammite be given in marriage to your brother Adonijah.”

 ²² King Solomon answered his mother, “Why do you request Abishag the Shunammite for Adonijah? You might as well request the kingdom for him—after all, he is my older brother—yes, for him and for Abiathar the priest and Joab son of Zeruiah!”

I did a little research and this was not a case of thwarted love ,although  Adonijah probably hoped Bathsheba would think so.  Although she was still a virgin, Abishag was considered part of David's harem.  Adonijah wanted Abishag because possessig the king's harem was equivalent to claiming the throne.  Absalom had done the same thing in his rebellion against David (2 Samuel 16:20-23).  Solomon well understood what Adonijah was trying to do.

¹ When the time drew near for David to die, he gave a charge to Solomon his son. ² “I am about to go the way of all the earth,” he said. “So be strong, act like a man, ³ and observe what the LORD your God requires: Walk in obedience to him, and keep his decrees and commands, his laws and regulations, as written in the Law of Moses. Do this so that you may prosper in all you do and wherever you go ⁴ and that the LORD may keep his promise to me: ‘If your descendants watch how they live, and if they walk faithfully before me with all their heart and soul, you will never fail to have a successor on the throne of Israel.’

What a legacy for a father to leave his son. "Act like a man and observe what the Lord your God requires! If only more fathers instructed their sons in such a way... Even though David wasn't a sinless man, he remained faithful to the one true God of Israel.  When he sinned, he repented and found forgiveness as he turned his heart back to the LORD. One sin that David did NOT commit was idol worship.

Solomon had a great beginning. He firmly established his kingdom against all his enemies, the LORD gave him wisdom, wealth and honor. What was Solomon's downfall? He married foreign women who turned his heart to other gods.  As Solomon grew old, his wives turned his heart after other gods, and his heart was not fully devoted to the LORD his God... The LORD became angry with Solomon because his heart had turned away from the LORD, the God of Israel, who had appeared to him twice. Although he had forbidden Solomon to follow other gods, Solomon did not keep the LORD’s command. (1 Kings 11:4, 9-10)   Because of this sin, the Lord tells Solomon that he will take tear the kingdom away from his son.  When Solomon's son Rehoboam comes to power, the kingdom is divided as the LORD said he would do. Our God is a jealous God. He will not be worshipped along side other false gods....but it appears that the LORD was gracious to even Solomon, for as he begins the book of Ecclesiastes with  “Meaningless! Meaningless!” says the Teacher. “Utterly meaningless! Everything is meaningless.” (1 Ecclesiastes 1:2), he ends the book with "here is the conclusion of the matter: Fear God and keep his commandments, for this is the duty of all mankind. For God will bring every deed into judgment, including every hidden thing, whether it is good or evil. (Ecclesiastes 12:13-14) The man who had everything (money, women, power) says everything is meaningless and that the one thing that remains is to fear the Lord and obey his commands.  Isn't that exactly what his father David told him even before he became the biggest and the greatest king on earth?

How far will we fall before we learn the same life lessons that Solomon came to realize at his old age?

I had the same question as Fernando and read your explination.  But it said that Abishag had no sexual relations with David.  I guess that didn't matter?  The rest of your explaination holds up regardless of whether she was a virgin or not, just curious I guess.

Also, there was a lot of death at the beginning of Solomon's reign.  When he kills Joab, he says that his father 'knew nothing about the deaths of Abner son of Ner, commander of the army of Israel, and of Amasa son of Jether, commander of the army of Judah."  But in Chp2v5, David tells Solomon the story of what happened.  Was what Solomon said simply pomp and circumstance and providing an excuse to kill Joab?  Why did Solomon specifically say his father knew nothing about them?  Maybe starting to set the stage to show how "wise" he is (will be)?

My other question is why did David charge Solomon with carrying out these executions?  Why didn't David kill Joab when he first learned of the murders?  Why let Shimei live only to order his son to kill him later?  And is the story of Adonijah "manipulating" Solomon's mother a foreshadowing of Solomon's future wisdom (not refuding his mother, but seeing the subterfuge of Adonijah, therefore not granting the request), similar to the statement above?

Yujin responds...With respect to Joab's murders, the expression "knew nothing about the deaths..." does not mean that David was not aware that Joab killed the two commanders, but that he was not aware of Joab's plots.  Now, regarding why he did not have Joab killed when he found out about the murders, the chief reason may be that David used Joab to commit David's own murder of Uriah. I'm sure that the guilt of his sin, which plagued David throughout his reign, weighed upon him with respect to Joab. Some have rightly suggested that David was somewhat afraid of Joab and felt powerless during his reign to go against this popular and mighty general. Finally, Joab was very loyal to David throughout David's life, and he did not join in Absalom's rebellion against his father.

With respect to Shimei, the text indicates that David swore to God not to kill Shimei; therefore, David was bound by oath not to harm Shimei by his own hands. Solomon had no such constraint; however, he wisely allowed Shimei to dig his own grave while preserving the reputation of his throne. 

With respect to Adonijah, Solomon's insight and actions does reveal his wisdom. Even though he would later pray for wisdom and receive it in greater measure, he was already quite wise. Even David acknowledges this about his son (1 Kings 2:9).

Why did adonijah get killed for asking for abishag?

Yujin responds... Great question, Fernando! Solomon saw through Adonijah's plot. Abishag was not David's wife but she was like a concubine to him, and so part of the royal harem. If you recall, previously David's other son, Absalom, slept with David's concubines in order to solidify his hold on the throne against David when he usurped David's throne. Adonijah may have had a similar plan. Also, since Abishag was with David until the end of his life, she would be an important witness as to whether David indeed pronounced Solomon king or not. If he could marry her, she would be beholden to defend her husband against Solomon. Adonijah's own words suggest a continuing resentment against Solomon: "You know that the kingdom was mine and that all Israel expected me to be king; however, the kingdom has turned about and become my brother's, for it was his from the LORD" (1 Kings 2:15). Also, rather than approach Solomon directly, he tried to influence Solomon through his mother, Bathsheba. Solomon saw the treason in both his goal and his approach. This is likely why he was killed. This is suggested in Solomon's reply to Bathsheba: "And why are you asking Abishag the Shunammite for Adonijah? Ask for him also the kingdom--for he is my older brother--even for him, for Abiathar the priest, and for Joab the son of Zeruiah!" (1 Kings 2:22).