Why do people say that suicide is selfish?

Seriously, I know that suicide hurts those who care about the depressed person, but at the same time if these people cared so much would they really want someone to live in misery solely to help their emotions? Mind you, I'm not talking about people who have never been treated, but those for whom mental health treatment hasn't eliminated a serious desire to commit suicide. Doesn't living life hurt them? In that case, who's really the selfish one? Think that someone should be allowed to commit suicide if they are terminally ill, or if they are severely depressed and after receiving treatment for a period of, say, 1 year they still want to die? Maybe those 21 and up? What are your perspectives? Why should someone have to exist solely to please another? I've heard the saying "Suicide is a permanent solution to a temporary problem", but depression isn't really temporary, at least not the recurrent kind - it comes and goes throughout life. I knew of a bipolar guy who poured a can of gasoline onto himself and lit himself on fire when he was 21. Even his parents thought his death was more merciful than him living with his mental illness on top of being severely disfigured and disabled would've been. But wanting someone to live in so much anguish, long term, that they think burning to death is better than life - Isn't that selfish?