Mice do many things by instinct. If they see a female mouse, they want to breed with her. If they see a male, they become get territorial and will pick a fight. Many of these tendencies are ingrained in mice and aren't taught - it's just part of who they are.
These are some reasons that a mouse will fight another mouse:
The male is encroaching on another male's territory.
They're fighting for a mate.
Sometimes mice play-fight. (This is more common with young mice.)
They're trying to establish dominance.
They're hungry and food is scarce.