A suicide index

Researchers have come up with a suicide index for rating how likely someone is to commit suicide. Apparently, being young, female, unemployed, and living alone are great indicators. Ahem. Anyway:

"In places such as the United States, for every death from suicide there are between three and four hospitalisations resulting from failed suicide attempts, and between 15 and 20 emergency room visits for the same reason."

That's a lot of suicide attempts. And a pretty high failure rate. Which is a good thing, but why do humans try to kill themselves? What makes us so self-destructive? Do other animals do it? Is it because we think philosophical thoughts? Other animals get depressed, why don't they try to kill themselves? I bet they do and we just don't know it.