Kittens use their mouths to explore their environments. It is not a show of aggression. Oftentimes, kittens are taught by their mothers or other kittens in their litters not to bite, but owners may need to take steps to stop biting.
Pet experts recommend expressing displeasure when a kitten bites and then immediately walking away and ignoring the kitten. This teaches the kitten that biting causes him to lose attention from the owner. Giving the kitten a treat when it doesn't bite and praising the kitten is also important. This teaches the kitten that not biting leads to attention and treats.