Why Do Sampson and Gregory Fight With Montague's Men?

In William Shakespeare's play "Romeo and Juliet," Gregory and Sampson fight in defense of their master. These two men are servants of the Capulet family, and there is a long-term grudge between the Capulets and Montagues.

As the fight occurs, Tybalt encourages them, but Benvolio attempts to stop them. The wives of the patriarchs of the Capulet and Montague families tell their husbands that old men do not need to be fighting, and they do not get involved. When the fight reaches its peak, the prince threatens death to those who are fighting. When the townspeople agree with his threats, both parties choose to walk away and not continue to fight.