I for one, am one who has had those kind of thoughts... and everyonce in a while I'm thankful they never really got anymore than that.
Let me just tell you that it's not even an easy thing to keep those thoughts in your head, it really does get overwhelming.
One of the main reasons I think one would be is because, to put it simply, they just lost the will to live. You go through life, the same routine and things seem to be getting a whole lot worse. You begin to think, "What good have I done?" "How would others people's lives be without me?" "Would anyone even care?" You start to convince yourself that things would be alot better off if you weren't there; you're just a burden to everyone around you. It really is a long thought process, and I think most people that do decide to take their lives have been thinking about it for months, or even years.
From what I also noticed, some begin to feel suicidal when some kind of traumatic event occurs in their lives. Alot of times the person who commits suicide is mentally ill.
There are several reasons why people do it. It's not always simpley because they just don't want to deal with their lives. This was just a general thing. I believe that nobody will ever know the exact reason why someone decides to kill themselves because we all have different things going around our heads.