He remembered a pledge he made that when he became king and had peace from his enemies, he would treat Jonathan's family with kindness. If you owe someone a favor, it is better to do it late than not at all. David was a man sitting there thinking what he could do to help people. Thinking of the needs of others. He lived to do good.
Good men seek opportunities to benefit others.
David did not just sit at his desk waiting until someone came up to make an application for help. David's love was unconditional of merit. David had forgiven Saul and now wanted to show kindness to Saul's family. This man after God's own heart overcame evil with good. David was compassionate.