Due to the ongoing feud between their families, the Montagues and Capulets, when Romeo accidentally murders Tybalt, his cousin and Juliet's family member, in a street fight.
The Prince of Verona subsequently banishes Romeo for his act of violence, effectively prohibiting him from seeing Juliet ever again.
To Romeo and Juliet, this separation feel like the ultimate tragedy, driving much of the emotional anguish and dramatic developments during the play. They desperately attempt to find ways to reunite amidst their forced separation, resulting in the unfortunate series of events and their dramatic demise (Romeo mistakenly believing Julietis dead and drinking poison.)