I just (hopefully) came out a low point in my life recently, so the memories are still fresh.
In my case, it was several things that added to one depressing outlook. Interestingly enough, most of those were lies made up by my own mind during the time. It just had a way of repeating the lies until they felt true.
I felt as though I wasn't needed, so my death wouldn't have a large impact on those I love. Also, I felt that my future wasn't worth waiting for (school, school, school, job, job, job, death). Finally, the general environment at the time was just unhappy.
Now, with a happier perspective, I could tell the first two were kind of imaginary. My mind had managed to stretch small things (i.e. financial stability) to huge issues (i.e. they don't need you). The first two were basically results of that.
I'm not a professional, so take my advice with a grain of salt:
What really brought my out of my depression (I don't want to say depression, because I don't have it) was finding out that my self-told lies were really lies. I'd say clearing up any misunderstandings by the suicidal person can really help if they harbor misunderstandings.
Of course, getting a professional is definitely a good idea.
Funny how easy it is to talk about these things on the internet.