I think laughter is a good thing. I'll sit there when I'm alone and laugh like an idiot. I do this because I'm a jukebox, and sing whatever comes into my head. It's actually very therapeutic.
Also, I find that exaggerating my laughing response just a little helps around other people. It's not that I laugh at things that aren't funny, I just laugh a little harder. I never ended up doing it on purpose but I noticed myself doing it one day, it was a product of a change that happened in my life. When it happens part of me is always like
but I find that I don't care much as long as I'm in a better mood. I'll take it.
Still, I agree in that some people who laugh are annoying, especially when they're completely non-responsive to myself, it's just a minor slap to the ego when I see them laughing with someone else.
Given that I have depression and try to laugh, you may be wrong about other people that you see. Everyone, even people who aren't clinically depressed, probably have something in their life that gets them down continuously, too. This doesn't mean they have it better or worse, I guess it's just something to consider.