It's not Juliet that Romeo loves at the start of the play – it's Rosaline. This makes Rosaline an obvious foil for Juliet, so that Romeo's relationship to Juliet (the way he describes her and acts towards her) can be contrasted with his puppy love for Rosaline.
Rosaline has no lines in the play and never appears on stage (according to the stage directions) but we hear a lot about her, specifically, how much Romeo loves her and how she doesn't love him back. In fact, Rosaline has made a pledge to remain "chaste" (1.1.13). This makes her a good foil for Juliet, who is an individual in her own right and has a lot to say. She's also very responsive to Romeo's passion and she makes no apologies for her sexual desire.Over the years, directors have made some interesting choices in casting the roles of Rosaline and Juliet to play up the differences between the two women. In Zeffirelli's 1968 film adaptation of Romeo and Juliet, Rosaline is lovely but she's also very stiff and at least a decade older than Romeo. Zeffirelli's Juliet, in contrast, is a young, lively, mischievous beauty who can't keep her hands off of Romeo.
Juliet is originally supposed to marry Paris, and whatever Romeo's faults may be, Paris serves as a foil to make Romeo look pretty good. It's hard to question Juliet's judgment in preferring Romeo to Paris. After all, Paris is always "convention" personified. He asks for Juliet's hand in marriage because he is friends with her father and because she's from a respected family with money. Paris's behavior at the end – putting flowers on Juliet's grave like a conventional grieving lover – is totally consistent with his attitude toward Juliet throughout the play. Although Romeo has a few moments where he acts like a cliché lover, his desire for Juliet is authentic and passionate and he falls for Juliet before he even knows that she's a Capulet.
Director Baz Luhrmann goes out of his way to play up the differences between Paris and Romeo in his 1996 film, Romeo + Juliet. In the film, Paris (played by Paul Rudd) is the cheesy and buffoonish suitor that acts as a foil to the earnest and passionate Romeo (played by Leonardo DiCaprio.) Check out Paul Rudd ham it up as Paris in this clip of the costume ball scene. FYI: In the play's script (the thing you read in class) Paris never actually shows up at the ball, even though Juliet's mom tells her daughter she should check him out at the party.