Albert Barnes Commentary 1 Samuel 10:1

"Then Samuel took the vial of oil, and poured it upon his head, and kissed him, and said, Is it not that Jehovah hath anointed thee to be prince over his inheritance?" — 1 Samuel 10:1 (ASV)

Is it not because ... - Samuel answers Saul’s unspoken or expressed wonder by explaining why he acted as he did. (Compare 1 Samuel 9:21.)