the reason, i think, is a dichotomy that plagues our search for satisfaction. either they're generally depressed people with generally unhappy lives (or maybe lives that could easily be parsed as unhappy), or they're happy people whose happiness is largely effected by things (usually people) whose absence could crush them.
i don't know that there's a good way out of this. the moment things start to matter to you, the moment that things become a source of happiness, is the moment you open yourself up to the cliffy downslope which results in hitting the ground hard. if you're generally miserable, your life at least isn't going to get much worse, which may be why this option is so attractive. the solution, obviously, is to simply have a lot of different things that make you happy so that no one will destroy you with its dissolution.
the problems with this are numerous, however. first of all, happiness is a pretty relative concept, and it's easy (at least for me) for the best option to simply trump out the happiness of all the others. secondly, you only have so much time (even less if you have a real job) to divvy up, and it's hard to fit a large number of disparate activites into it. thirdly, to some extent orthogonal activities require orthogonal personas, and it's hard to maintain that many faces. the easy way out is to find one thing and sink your life into it, potentially ending you up with a gaping void.
i'm not sure if the suicide thing has to do with the people i know in particular, who i suspect are certainly more depressed than the everyday population. this is both correlated to the ways in which i get to know people and to the fact that i find people with problems more interesting, and i'm after interesting people. in any case, most of the people i know are certainly those who would take the easy way out, which might have something to do with it.
zen buddhism, which i took a class in last year, teaches that most forms of happiness are impermanent, and that it's important not to get too attached to anything worldly. they up the ante by postulating a more permanent form of happiness in nirvana, but without crossing that line it seems like the paradox they concoct about human existence is not without its truth. anything that brings you happiness can go away; you may even get bored of it. so either you're hopeless or disappointable.