Charles Ellicott Commentary 1 Kings 2:15

"And he said, Thou knowest that the kingdom was mine, and that all Israel set their faces on me, that I should reign: howbeit the kingdom is turned about, and is become my brother`s; for it was his from Jehovah." — 1 Kings 2:15 (ASV)

You know.—Adonijah’s words reveal his craftiness. He flattered Bathsheba by praising her influence and presented his surrender to Solomon as a meritorious act, claiming it was in obedience to the Lord’s decree regarding a kingdom that, in clear contradiction to the facts, he asserted was destined for him by popular desire.

The petition, however, seemed harmless. Since Abishag was David's concubine only in name, it involved nothing unnatural. Yet it perhaps concealed a hidden motive, for by universal Eastern custom, taking a king’s wives was the known privilege or duty of his successor. This custom explains the counsel—most improper, yet still probably strategic—given by Ahithophel to Absalom (2 Samuel 16:21).

Therefore, if Adonijah had publicly married Abishag, it might have seemed like a virtual renewal of his claim to the crown. Solomon understood this immediately, though Bathsheba, strangely enough, did not.