If you tickle a rat, it can laugh, by making a chirping kind of sound. Rats, however, don't seem to be social gigglers like humans are. Humans are most likely to laugh when with other people and unlikely to laugh when alone.
Laughter is not all about humour though; laughter has a very dark side. Aristotle and Plato both commented on laughter as being an attempt to assert superiority. And psychological research also shows that socially dominant children are more likely to laugh more often. Also consider that the kids who perpetrated the Columbine School massacre were laughing as they engaged in their repugnant shooting spree.
Babies begin to laugh at around 2-3 months of age and playing with a young baby can soon develop into joyful laughter for all concerned and the release of oxytocin, a brain chemical that increases feelings of attachment. Laughter in this way can be a serious form of social and emotional bonding and connectedness. It's pretty hard to fake genuine laughter, though, in the same way, that it's hard to fake a "genuine" Duchenne smile.
Laughter is often contagious, you seem to catch it from others. To some extent, this is because we tend to mirror or mimic, the behaviour and gestures of others who we interact with. We also release feel-good natural brain opioids, called endorphins, when we have a laugh and this makes us feel good and little bit "high" too.
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