I guess the best reason is because hopefully you've experienced 5 minutes of happiness at least once in your life...and it's logical that the conditions that brought you that happiness and be reproduced.
I was very suicidal last night and couldn't find a reason to keep living but i didn't want to die either. but felt i had no choice. So I took 3 sleeping pills and went to bed in hopes that i'd wake up in the morning with at least one reason. I still am but not as bad as i was last night.