When old people "lose the will to live" it's really just a combination of old age and probably a tendency to not take care of themselves like they once did. When you're really old, the likelihood of dying soon is pretty good anyway. Being wracked with depression only compounds the problem because you're not likely to do things that are going to keep you vibrant and healthy. So really you can still break it down to a physical reasoning for death.

What's Padme's excuse? There is none. And the fact that her death completely flies in the face of the character that's been built up by 5+ hours of film makes Lucas's writing all the more embarassingly bad.