If it was that simple then suicide wouldn't even be an issue.

You ever heard the old cliche': He seemed fine, no one could have guessed what was going on in his head... It's said for a reason. Of the few people I know who have committed suicide, none of them seemed like they needed any help or assistance.

Add to the fact that depression is such a misunderstood condition that I'd imagine most people are unwilling to "out" themselves as much of the time it would get the wrong reaction.