Act I, scene i of the play sets the stage for all future actions. Shakespeare provides all of the background information one needs to understand the play.
Sampson and Gregory (Capulet's) are walking in the streets of Verona and expressing their hatred for the Motague's. They provoke a fight with two Montague's that the see. Sampson bites his thumb, a highly offense gesture, at them. The verbal fight quickly escalates to a physical brawl until the Prince shows up and declares that there will be no more fighting on his streets. This fight shows the importance of masculine bravado. From the very beginning, we know that there will be future fights. One of my favorite lines in the play:
"Here's much to do with hate, but more with love" (I.i.175) is also an indication of what is to come.
Also, this is the first time we see Romeo as a "lover". He is introduced as a love-sick young man. However, it is not Juliet that he loves. He loves Rosalie. He is very passionate saying things such as " Feather of lead, bright smoke, cold fire, sick health” (I.i.173).