Because I tried it and failed. You know your life is really sh!t and you've really hit rock bottom when you can't even succeed at that!
But basically, it did mean that however bad I felt at that particular moment in time, I've since gone on to live through some really wonderful times, things took a turn for the better, yeah, things haven't been all plain sailing, I've also had some more bad times, but know that when you're absolutely convinced - as I was - that things are really bad, and they'll never, ever get any better... to find out that I had been wrong.
Well, if I had succeeded, I would never have known I was wrong, I would never have known that things would, in fact, get better. And even though I was convinced it was so, my friends wouldn't actually have been better off without me. In fact, I found out who my true friends were and are.
The lesson I learned, and which I would like to pass on to anyone considering it is: Don't. Don't do it. No matter how bad you think things are, there is always a way to get round or get over whatever problem seems to have driven you to this point. And in a year, or five or ten years, when you're looking back on this point, and you've lived through some wonderful experiences, met some lovely people, you will be so relieved, so glad that you didn't do it.