They make me laugh.
True, other people and things can make you laugh, but nothing and no one can make you laugh like your kids can. They can do it by making a funny face, saying a certain word or phrase, showing off a new-found sense of humor, constantly falling over their own feet, doing something that makes you so proud that a smile just isn't enough, or just laughing themselves. Yes, they can also make you so frustrated that you cry, so upset that you scream, and make you wish that you could lock yourself in a room where no one is allowed to talking (or at least not allowed to say "Mommy!"), but when they make you laugh, it makes all of those not-so-good times worth it a thousand times over.