Homo sapiens is, sadly, a species that enjoys sameness. Anything that sets an individual apart and makes them different is shunned to some degree by the average person.
That's why people who are outside the norms are poked fun at, be it because of a conspicuous mole, their weight, their height, their hair, or their facial features. That's why we have racism. That's why we think people who aren't part of our social clique are "weird".
Animals like their own kind to be the same, and anything that sets one apart from the others could mean that they are weak or a danger to the group. We are after all, animals. It's sad that humans, for all our evolutionary gifts, can't overcome our basest of instincts. We have all done it at some point or another, whether it's cringing at a scar or laughing at the tall skinny kid on the playground.
That doesn't make it right, we are intelligent enough to know that it's wrong, and we should all do our best not judge others for being different.