I wouldn't like to view it as a rational choice either - since life is full of potential like you said - but that's how we think of it.

It could be said another way around as well, that it's hard to imagine such pain in happiness. Ultimately, because pain and happiness are subjective metrics and there's no objective measure to estimate them, I guess it's impossible for any of us to have total agreement on whether it's justified or not.

I think the last point is interesting though - many who have survived suicide attempts are now happy. How does the change happen? since that's the key to pull people from depression and alter the situation into a better direction.