When puppies encounter mirrors, they are not really sure what to do. Is there another dog behind this strange reflective wall, or is it a ghost? Puppies which encounter mirrors for the first time cannot really be sure. But all their fights against the mirror will maybe give them the insight that it is not really necessary to fight the ghost puppy! They might bark sometimes, but they are not really angry, just afraid of the unknown. Because, as well all know, small dogs usually just want to play. This is not different when puppies see themselves in a mirror.
source