The Montague and Capulet families are feuding. Unfortunately, they are two quick to fight, so the feud continues.
Gregory and Sampson are young and ready for a fight. They do not care why they are fighting, they just know that they are affiliated with the Capulet family and therefore they are supposed to hate the Montague family.
But thou art not quickly moved to strike. (Act 1, Scene 1, p. 9)
Sampson notes that he is moved by a “dog of the house of Montague” (Act 1, Scene 1, p. 9). This is the problem. The Montagues and the Capulets are always ready to fight, so they keep killing one another and extending the feud.
This line does not only describe the scene, but the whole play. The two families are feuding and will continue to feud until their strife is buried by their children’s death.