David takes a great risk in showing kindness toward Saul’s grandson. Mephibosheth could have been considered the heir to Saul’s throne and therefore a threat to David. Instead, David treats him as the son of his best friend rather than as an enemy.
David valued relationships over personal power. Sometimes people get that backwards. The pursuit of a promotion or a better job can cause family and friendships to be pushed aside. These changes might make a person rich and powerful, but empty, untrustworthy and lonely as well.