Albert Barnes Commentary 1 Kings 22:29

"So the king of Israel and Jehoshaphat the king of Judah went up to Ramoth-gilead." — 1 Kings 22:29 (ASV)

One might have expected Jehoshaphat to withdraw from the expedition when he heard Micaiah denounce it. He had, however, rashly committed himself to the war with a solemn promise before he thought to inquire what the will of God was in the matter.

Now he was ashamed to draw back, especially since Ahab, whom the prophecy chiefly threatened, was resolved to brave it. He may also have had a personal affection for Ahab and was therefore reluctant to desert him in his need (Compare to 2 Chronicles 19:2).