God rejected Saul as king because of his refusal to obey and trust Him.  Instead, God turned to " a man after His own heart" and anointed him as king.  David did not receive the kingship immediately, though.  He had to wait.  As Saul's spirit darkened and as he pursued David's life more vigorously, David learned to put his trust in God, to wait for God's promises, and obey even in the worst of circumstances.