For I will surely show you kindness for the sake of your father Jonathan

2 Samuel 9:1-13

David asked, “Is there anyone still left of the house of Saul to whom I can show kindness for Jonathan’s sake?” Now there was a servant of Saul’s household named Ziba. They summoned him to appear before David, and the king said to him, “Are you Ziba?”

Ziba answered the king, “There is still a son of Jonathan; he is lame in both feet.”

When Mephibosheth son of Jonathan, the son of Saul, came to David, he bowed down to pay him honor.

“Don’t be afraid,” David said to him, “for I will surely show you kindness for the sake of your father Jonathan. I will restore to you all the land that belonged to your grandfather Saul, and you will always eat at my table.”

Then the king summoned Ziba, Saul’s steward, and said to him, “I have given your master’s grandson everything that belonged to Saul and his family. You and your sons and your servants are to farm the land for him and bring in the crops, so that your master’s grandson may be provided for. And Mephibosheth, grandson of your master, will always eat at my table.” 

Then Ziba said to the king, “Your servant will do whatever my lord the king commands his servant to do.” So Mephibosheth ate at David’s[a] table like one of the king’s sons.

Why?

David did not forget the covenant made with Jonathan, David took care of the living of Mephibosheth like one son of David. David showed kindness for Jonathan's sake.

David returned all Saul's belongings to Mephibosheth and asked Ziba and his family to serve Mephibosheth and his properties.

How?
David is a trustworthy man, so the Lord returns the mercy to him either.