David was out humbly doing his duty. With seven older brothers, he never thought about being first in anything. Human wisdom would give him about as much chance of being chosen king as the third monkey on the ramp going into Noah s ark. Jesse was not the best father. He showed partiality among his sons.
Many parents make the same mistake.
He did not treat his children equally. David was the most unlikely choice. Yet he was God's first choice. God takes nobodies and makes them into somebodies. God had done His homework.