Juliet doesn't want to marry Paris because she is already married to Romeo. Additionally, she is deeply in love with Romeo and cannot conceive of marrying anyone else. When her parents tell her that they plan to have her married to Paris, Juliet begs and pleads with them not to force her into a marriage that would bring her so much pain and unhappiness. She says that she would rather die than be separated from Romeo, and she threatens to if she is forced into a marriage with Paris. Juliet's refusal to marry Paris sets off a chain of tragic events that eventually leads to her and Romeo's deaths.
