David came to Saul and entered his service.1 Saul liked him very much, and David became one of his armor-bearers.
References for 1 Samuel 16:21
Then Saul sent word to Jesse, saying, "Allow David to remain in my service, for I am pleased with him."
Whenever the spirit from God came upon Saul, David would take his harp and play. Then relief would come to Saul; he would feel better, and the evil spirit2 would leave him.
References for 1 Samuel 16:23