2 Samuel 18:5 records David's command to deal gently with Absalom, revealing the love that now fuels his mourning.

In 2 Samuel 18:14, Joab kills Absalom — the direct cause of David's mourning reported here.

In 2 Samuel 18:20, Joab prevents Ahimaaz from bringing the news because the king's son is dead — showing Joab knew David would mourn.

In 2 Samuel 18:33, David's cry 'O my son Absalom' gives the actual words of the mourning described here.

Proverbs 17:25 says a foolish son brings grief to his father — a general principle illustrated by David's grief over Absalom's rebellion.