Because the Lord was with David but had departed from Saul

1 Samuel 18:1-30
Jonathan became one in spirit with David, and he loved him as himself. From that day Saul kept David with him and did not let him return home to his family. And Jonathan made a covenant with David because he loved him as himself.
Saul was very angry; this refrain displeased him greatly. “They have credited David with tens of thousands,” he thought, “but me with only thousands. What more can he get but the kingdom?” And from that time on Saul kept a jealous eye on David.
Saul was afraid of David because the Lord was with David but had departed from Saul. So he sent David away from him and gave him command over a thousand men, and David led the troops in their campaigns.
When Saul realized that the Lord was with David and that his daughter Michal loved David, Saul became still more afraid of him, and he remained his enemy the rest of his days.
Jonathan's instant liking deepens into lasting friendship with David, who will look back on this as one of the best things in his life. Nothing could shake the amazing bond between the king's son and the man who, humanly speaking, was to rob him of the throne. 
As David's prestige grows, Saul's Jealous suspicion increases, and he plots David's death. David's poverty gives Saul the chance to suggest a bride-price which is likely to cost him his life. Such a toll could be exacted only from the Philistines. Israel's chief enemy: the other nations practiced circumcision. David doubles Sahl's demand and returned to claim his total bride.
Why?
Because of the Spirit of God is with David, so David led the troops in their campaigns. 
Saul could not better than Davis so he remained his Jealousy on David the rest of his days. 
Jesus Christ is my savior, even there are many difficult things but God is my fortress and my strength and my way.
How?
What can I do is obey God's command and follow His way with all my soul, all my heart and all my strength. Then if God permits, the Spirit of the Lord be with me.