When Saul saw that David was very successful, he was afraid of him.
Saul feared David because he sensed that God had left him and was with David instead. Note this well: those truly great and to be reverenced have God with them. David's wise behavior only deepened Saul's fear, for wisdom commands respect where threats and hectoring earn only the contempt of the discerning. Saul foolishly removed him from court, thinking to weaken him, but gave David instead a chance to win the people's love.
AI summary
Commenting on 1 Samuel 18:12-30
And all Israel and Judah loved David,.... The verb is singular, and denotes that everyone of them loved him in all the tribes of Israel, as well as in Judah his own tribe; in such general esteem was he, and so much had he got the hearts and affections of the people: because he went out and came in before them; the people, as in...
He was afraid of him - He saw that, by his prudent conduct, he was every day gaining increasing influence.