"My son," said the star, "it would be no use, even though you wished it, to sail for the World's End with men unwilling or men deceived. That is not have great unenchantments are achieved. They must know where they go and why." (14.27)
Caspian is a king, but he's not supposed to be a tyrant. As Ramandu reminds him, he governs with the consent of his subjects – especially when it comes to dangerous adventures!