Albert Barnes Commentary 2 Samuel 10:2

"And David said, I will show kindness unto Hanun the son of Nahash, as his father showed kindness unto me. So David sent by his servants to comfort him concerning his father. And David`s servants came into the land of the children of Ammon." — 2 Samuel 10:2 (ASV)

The history does not record any instance of Nahash’s kindness to David, but the enmity of the house of Nahash against Saul may have disposed him favorably toward Saul’s enemy, David. Furthermore, if there was any family connection between David’s house and Nahash (2 Samuel 17:25), this may have increased the friendship.