Humans find kittens cute due to their biologically similar features to infant children. The act of anthropomorphizing small animals, such as kittens and puppies, is common to most people, leading to a natural feeling of protectiveness or control over the animal.
Cuteness (primarily identified by large heads, big, round eyes and tiny noses) is evolution's method for insuring that an adult takes proper care of its offspring. The human mind latches on to cute qualities in many species, from human babies to kittens, puppies and pandas. As with babies, a young, helpless and cute face on an animal can lead to a sense of nurturing in the onlooker.