Howl cannot fall in love because Calcifer has his heart therefore he cannot give it to someone else, but when he does fall in love the Witch will find his heart and take it for herself and therefore killing him.
Howl cannot fall in love because Calcifer has his heart therefore he cannot give it to someone else. However, has part of the curse the Witch has put on him, when he does fall in love the Witch will find his heart and take it for herself, and therefore killing him.