Their frown.
Creepers are sad and depressed creatures. They are very suicidal and homicidal too. That would explain the frown on their faces. They are always looking for ways to kill them selves. Their creeper friends are just to sad too feel happy. They're just looking for a reason to blow up. They find a Steve and try to blow him up because monsters are taught to attack people. They think "Hey I can kill him and kill my self too." Sometime they get their friends and kill each other just to kill themselves. Now excuse me I hear something at my door...