Look at it this way: we are probably half right if we say to ourselves that the rest of lives is worth living, or that it is not.
If you have decided that your life is not worth living, then the next thing to do, if you are rational, is to make your death worth dying for.
This is how the mind of the suicide bomber works.
I imagine ISIS know this, and target the young with just this in mind, offering them posthumous honours if they do their dirty work.
I know many young people who suffer from depression and openly admit it. I vividly remember these feelings too, which seem to fade as one gets older.