1 Samuel 15:30 Saul replied, "I have sinned. But please honor me before the elders of my people and before Israel; come back with me, so that I may worship the LORD your God."31 So Samuel went back with Saul, and Saul worshiped the LORD.
Did you notice the phrase "your God" in Saul's words? I did. Saul was the man the people chose, but he made excuses for his disobedience (see verses 9-15), made rash commands that put his army at risk (remember in chapter 14 he vowed to kill his son for eating honey? His army was weak from lack of nourishment), and was great in his own eyes (1 Sam. 15:12 states he made a monument in his own honor).
There is more to that passage I quoted above, too: Saul was interested more in his standing among the people than his standing before God (verse 30).
Saul served and worshiped God, but did not draw near enough to call him "our God" or "my God." So, God chose another: David. 2 Samuel 22 (also Psalm 18) shows David repeatedly calling God, "my God," "my Lord," and so on. David made mistakes, but he did not make excuse for them. He repented out of a truly sorrowful heart. God called him a man after God's own heart (1 Sam. 13:14; Acts 13:22).
Both men worshiped God, but only one was a friend of God. How is your relationship with God? Is he "my God" or "your God?"