Why does my dog get aggressive around puppies?

My (neutered) 3 year mutt tends to become a little aggressive around younger dogs and puppies. When we are at the park, if he sees a younger dog or puppy come in, he will seek it out, and chase it until it does the "I give up" rollover, then my dog usually will start biting the dog around the neck and bark at it. I usually intervene if the puppy starts crying or the owner starts getting upset, but if it gets to this point, it's usually too late. He'll turn into a puppy-seeking missile, and go after it time and time again, even if I discipline him. Any ideas why he does this? Any advice for keeping him in check? Thanks!