Almost all mammals will fight for two main reasons: fear or dominance.
Fear is part of the “fight or flight response”. When something threatens us we will either run away or stand up to it with aggression. This is a basic behaviour that we evolved to protect ourselves. You can see cats to this all the time, when a dog barks at them they will often stand their ground and hiss and spit in response.
Dominance is usually shown by males and is about showing other males who is boss, so you can get more stuff. That “stuff” may be more mates, more followers, more space or more food. This also evolved to help us survive. You can also see this in male dogs, who will fight for dominance until the “pack leader” has been established.
Most examples of human fighting usually comes down to the same two basic reasons.