Samuel arrives in Bethlehem under the cover of being there to sacrifice an animal. He invites Jesse and his sons to join in sacrifice. Samuel then looks over Jesse's sons one by one to find the one God has anointed king. He looks at each son until he has reached the end and does not find the one God has anointed. Samuel asks Jesse "Do you have any more sons?" Jesse tells him that, yes, there is one more son tending the sheep. David was the youngest son, he was beautiful in appearance, and was a musician. When Samuel saw David, God told him to anoint him king.
Without the spirit of the Lord, Saul found himself tormented. He was tormented to the point that his servants suggested that he get someone who can play music for him - that perhaps it will soothe his soul. His servants suggested a shepherd boy named David who they'd heard could play beautiful music. So unknown to Saul, he begins a relationship with the person who is set to take his place as king. Saul came to love David and made David his armourbearer - which was a position of favor.