Discuss the sources of tension in the first scene. Why are the characters upset?

In the first scene two Capulet servants – Sampson and Gregory – loiter on the street, waiting for some Montague servants to pass. When the Montague servants – Abram and Balthasar – arrive, Sampson bites his thumb at them (which is rude but not illegal). Insulted, Abram confronts Sampson and a fight begins.