There is no philosophical reason why we should not just commit suicide. Actually, it is quite easy to build a case that it would be better to commit suicide than to stay alive.
Anyway, self-cognition lets us realize that we anyway occupy the surface of the earth only for a short moment in time (as we do not yet possess the technologies to engineer an indefinite human life-span), and that thereafter, we will be dead forever. Thus, it doesn't matter whether we will have lived a bit shorter or a bit longer.
Furthermore, as long as we live, our potential for suffering is much larger than our potential for experiencing joy. It is so easy for pleasure to become boredom. But prolonged pain continues to hurt.
Logically, it would make sense to commit suicide, and philosophically, it could be a wise thing.
If only we could be guided by what we consider wise.
But we are just not built to commit suicide; it contradicts our genetic make-up. It would negate the reality that only those forms of life are born, the parents of which had a strong aversion against the logical solution of committing suicide. And this idea is carried in us genetically.
We have an enormous psychological barrier against terminating our lives: a psychological barrier that is very difficult to overcome even by those who are guided by the recognition that being dead at this very time would be preferable over being alive.
***
The most intelligent book on suicide, actually even advocating suicide, is Jean Amery's "On suicide". On Amazon.com, you can read through a number of pages.
On Suicide: A Discourse on Voluntary Death
http://www.amazon.com/Suicide-Discourse-Voluntary-Death/dp/0253335639
Jean Améry: A Biographical Introduction
http://www-english.tamu.edu/pers/fac/myers/amery.html
