There was a presentation about this somewhere on youtube. Basically it's a combination of feeling of being a burden to others + loneliness + fearlessness of death, courage. There are all kinds of different factors and different cases, but these are shared by all of them, regardless of whether it's schizophrenia, depression, or whatever.
Then it is broken down more. Loneliness and being a burden to others, self hate, lack of reciprocal care, stuff like that. This creates hopelessness, and a desire to die, but this is not enough. The person needs lowered fear of death. Still not enough. The person needs increased pain tolerance and courage too to do the act. Some of these qualities are given, and some can be learned or acquired.
The presenter also suggests that in the majority of cases suicide victims are not under the influence of any substance. Even alcoholics quit 3-4 days before. It's a hard thing to do, and can't be done while being intoxicated.