That’s an excellent question, not just because it impacts a key relationship in the film of Howl’s Moving Castle but because it also gives us an example of the contrast between relationships with an equal power balance and relationships where all power is given to one side. I’m not going to talk about this in terms of the relationships in Diana Wynne Jones’ original book because the film diverges so widely from its source – we’re really talking about different characters and relationships.
Howl and Calcifer exchange hearts impulsively, after a chance meeting at a moment of great importance for them both. Howl, still a boy with enormous ambition but not much experience of the world, was studying magic when he met a dying fire demon. He saw a chance to gain enormous power by keeping the demon alive at the cost of his own heart. He was at an age when his heart wasn’t of great importance to him, especially when he thinks it won’t endanger his life as long as the bargain holds. So Calcifer agreed to serve Howl in exchange for the chance to go on living, and Howl agreed to give up his heart in exchange for the continued use of Calcifer’s power. Neither was making the bargain out of altriusm. Neither considered the hidden cost – the impact of heartlessness and disengagement from humanity turning Howl into a monster over time.
So the significance of Howl’s exchange of hearts with Calcifer is that it shows how a careless, ambitious boy makes a bargain that leads him to be a heartless, vain man, and how that man changes under the influence of a sincere and unselfish love. Calcifer too is changed, from a demon bound into servitude to a human to a beloved and vital member of a human family. Both needed Sophie’s help to unlock their bargain without killing either or both of them, and apparently without Howl sacrificing the power upgrade the bargain gave him. And through the witch, Miyazaki shows us that making bargains with your heart is a risky business that can cost you all your youth and power.