Mice are preferentially kept in stable, harmonious groups. Introducing newcomers to a group or removing one or more animals, disrupts the social structure, requiring it to be re-established, which can lead to stress and conflicts.
Dominant males are strongly territorial, even in the absence of females. This can give rise to serious fighting (see picture), which renders social housing of males virtually impossible in some strains. Cage enrichment, particularly the availability of hiding places, may sometimes diminish aggression.
Scented substances, pheromones, and urine markings play a role in social behaviour. Scents clearly influence the measure of aggression. Both the absence of (the smell of) nesting material and the presence of urine/feces scents can lead to increased aggressive behaviour.