It may be because I live in a country without great access to mental health care (America, in case it wasn't obvious), but I went through a very long period of my life where I was horrendously depressed. Suicidal depressed.
I won't go into the details that led up to this event (I'm using a throwaway account for a reason!), but basically I had all of my dreams in life shattered and then I dealt with the Department of Justice for a year. The long-term stress robbed me of any self-confidence.
I finally got over it last year when I started doing small kind gestures for the people I care about. Volunteering to run errands. Cleaning the dishes. Anything to convince myself with concrete examples that people are happier that I'm around. Anything to justify not committing suicide.
And it worked, until last week when I suddenly started feeling anxious and depressed for no discernible reason. Being kind to others doesn't seem to have the same effect this time around, so before my condition gets any worse, I'd like to ask the community what reasons they have for continuing to live. Maybe I can figure out why I feel so crappy and pre-emptively stave off those inner demons.
The only reason I do keep living is that I find I can't answer the opposite question either:
Why not keep living?
At least for that variant many reasons do surface, and whilst none seem overwhelmingly strong they do allow the question to evolve into:
Why not keep living a little while longer?
A deferred decision as I've found no strong answers either way.
I suspect that living a long life is to just not have strong answers either way and to discover that time has passed and now you're old.
I don't buy the argument that suicide isn't an option, of course it is. When despair is all you have, and the future is bleak, it sometimes can appear to be the only option. Somehow though, a short time later, decision not made, I've always found myself in some place in which I'm glad I didn't. In which I've found some peace, some bliss, some joy from life... and doing so has driven the despair away.
I have no answer for "Why keep living?", but if you are willing to consider changing things in your life (to get away from the things that bring you despair) then you can find some answers of varying merit to the opposite question.