Mice is not a social animal. Although they can be found in groups, they do not actually live together like buffaloes or a school of fish. Having a varied diet means they are partly hunters too and such aggressive behavior can come out negatively if there is not enough challenges or activity in their life.
More so with humans. Although we crave company, those that truly seek large groups are more self-serving and violent than those who likes to be lone wolves. Example, party people v farmers. The farmers are more likely to help another in need than those who are nightclub owls and in case of an altercation, the party people would more tend to gang-up on a fallen while the farmers more likely to break the fight.

We are really not meant to live in a city environment. Not only is this generally unhealthy but it breeds violence and self destruction when there is not much productive activity to do.