At the beginning of the film, Howl gives Sophie a matching ring, saying if she's lost it will guarantee her safe return. Unbeknownst to her, it's magically connected to his heart. And though the movie never explicitly says so, I can't think of a better meaning for a wedding ring. https://twitter.com/tobiostars/status/1264481496817004545

Also, Sophie never explicitly breaks the curse placed upon her. It's just that whenever she's comfortable she gets her youth back (she doesn't even notice for most of the film). The curse isn't broken directly, she's just safe and self actualized all the time in her life now.

Also it's really cool how both characters visually transform through the film. Sophie's hair goes from dark brown to "the color of starlight" which suits her even better. While Howl reverts back to his natural black hair. Sophie grows. Howl gets his heart back.

A similar change in color scheme happens in Hayao Miyazaki's earlier film "Nausicaä of the Valley of the Wind" (1984). Towards the end of the film Nausicaä wears a pink tunic that gets dyed blue with the blood of the Ohmu she is trying to save in order to prevent further massacre

Physical transformation that reflects a deeper spiritual transformation is a hallmark of Hayao Miyazaki films.

Watch these films. Be inspired. Believe that you can change for the better! Be the best version of yourself! Be someone whose actions you respect!