A baby's laughter is adorable, flattering, and really kind of weird - what does a 2-month-old, or a 3-month-old, or a 4-month-old find so funny? And how, when she doesn’t even speak yet? It’s easy to tie verbal behavior to a sense of humor when you’re an adult, but your baby has to be basing all those giggles on something else, right?
Overwhelming anecdotal evidence - family stories from almost any parent ever - shows that a lot of what babies find funny is the exact same thing adults do: the unexpected. The unexpected isn't always funny, though - what's scarier than the unknown? Observations as far back as Charles Darwin’s own family stories about his children suggest that babies’ laughter and fear aren’t far apart, which is also a connection made by a 1989 study involving babies and Peek-A-Boo. That is, babies think the unexpected is funny when they don’t think it might be dangerous. That’s sensible, right? But because so many things are new to babies, it’s hard to say what new things will seem scary, and which ones will just seem hilarious. This can go both ways, too - sometimes that goofy noise you make to try to get a giggle out of your baby can just end up making her nervous. But on the other hand, things that you might think would ping her sense of danger, from a monster on a movie screen to that slapstick little fall you took in the kitchen last week, can just seem amusingly out-of-the-ordinary to your baby.
Basing a sense of humor on what’s unexpected means that what your baby thinks is funny, just like with what she thinks is scary, changes with her expectations of the world around her, and her expectations change as she learns. That means that as she learns what ‘normal’ is, she might be more likely to think it’s funny when things happen that are outside that range of normal. Of course, though, your baby is also an individual with her own sense of humor, which is going to continue to develop in the more specific direction of her interests as she grows.
So long answer? Babies laugh so much because humans find the unexpected funny, and babies have seen so little that they hardly ever know what to expect yet.
Short answer? Babies laugh so much because they're babies.