I think the original intend of laughter was to release tension and to show to others that they're safe with you, much like cats purr to show to their kittens that they're safe with the mother.
Laughter is, in fact, exhaling abruptly all the air you keep in your lungs, and then inhale a good quantity of fresh air. Sometimes it stops there, sometimes you repeat the process.
As for the making people feel safe part, I'm a bit confused about this, because once I read that dogs interpret smiling as baring your teeth, thus as aggressiveness. But I think the human perception is different. When you laugh, despite the fact your face muscles are tense, you appear less attentive to your environment, less observative and less in tune with the word around you. So you show to others that there's nothing to be afraid of and that they're in no danger when they are with you, cause you're not being very attentive at the moment.
Furthermore, we use to perceive as fake laughter the kind of smile that does not touch the eyes. This could be a sign that the person that is faking laughter is, in fact, still attentive to what's going on around them, so the no danger message they're trying to convey is just a distraction.
On a more on topic note, I laugh when something is funny and/or exciting.