I haven't read the book the film is based on, but from my understanding, the book has a more bittersweet or even sad ending. In it, metaphoric ownership of a star's heart isn't enough to make someone immortal. Tristan eventually dies of old age and then Yvaine returns to the sky alone.

Maybe this ending is more realistic (if realistic is something which can be applied to such a story). But I feel that a happy, slightly cheesy ending is more suited to a story which is basically a fairy-tale.