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 has a lot to say—including some unapologetic things about her own sexuality. Check out Rosaline's "Character Analysis" for more on this pairing.
Juliet is originally supposed to marry Paris, and, we have to say, he makes Romeo look pretty good. Paris is—sorry—a big old square, basically convention personified. He asks for Juliet's hand in marriage because he's friends with her dad and because she's from a respected, rich family. 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.
Meanwhile, Romeo falls for Juliet before he even knows that she's a Capulet. Okay, to our modern taste, falling for someone just because she's pretty might not sound particularly evolved—but it's a lot better than liking someone because her dad is influential, right?