As much as we hate to start our sentences with "as so-and-so said," we're breaking our rule here because this one is just too good. Here we go.
As scholar Marjorie Garber points out, all you have to do to see why Paris (the guy who wears down Juliet's dad until he agrees to let him marry Juliet) is such a good foil for Romeo – and why Juliet chose Romeo over him – is to contrast what Paris and Romeo each say outside Juliet's grave.
The obsequies that I for thee will keep
Nightly shall be to strew thy grave and weep. (5.3.2)
Paris's language says: "I'm a stiff and lacking in passion." There's no way that Paris would die for Juliet. He'll probably make other marriage plans as soon as the appropriate mourning time has passed.
The time and my intents are savage-wild
More fierce and more inexorable far
Than empty tigers or the roaring sea. (5.3.1)
Romeo, in contrast, is furious over Juliet's death, and eloquent in his fury. He won't drip a few tears on Juliet's grave and then go home to bed. Unlike Paris, this guy is a passionate lover.
An actor can make Paris seem like a total jerk, or like a sympathetic nice guy who happened to get caught in somebody else's love story. In Baz Luhrmann's 1996 film Romeo + Juliet, Paul Rudd plays Paris as a handsome but undeniably dorky guy. His dance with Juliet at the Capulet ball is very awkward – he starts swing dancing a little – and Juliet keeps looking at Romeo and making "this is awkward" faces. (We should point out that in Shakespeare's text, Paris doesn't actually show up to the Capulet ball as expected.) Check out Paul Rudd ham it up as Paris in this clip of the costume ball scene.
Paris's dialogue with Juliet in Friar Laurence's church can make him seem either a little clueless or a complete fool. Paris thinks Juliet is upset over Tybalt's death – he has no idea that she's already married to Romeo and that the prospect of marrying him makes her physically ill. It's easy to make some of Paris's lines seem overbearing and arrogant. He greets her with total confidence, "Happily met, my lady and my wife," he calls to her (4.1.3). Juliet and the audience cringe at this, but Paris keeps going. "Do not deny to him that you love me," Paris tells her (4.1.6). Later, when he looks at Juliet more closely, he tells her, "Poor soul, thy face is much abused with tears" (4.1.8). When she tells him that her face was bad enough to begin with, he reprimands her, "Thy face is mine, and thou hast slandered it," which hints at Paris's sense of ownership toward his fiancée (4.1.10). All of these lines together can make Paris come off as a spoiled boy who is used to getting everything he wants. His possessive attitude towards Juliet – especially laying claim to her face as his own – could come across as creepy and chauvinistic. It can seem like he's treating Juliet like another piece of property rather than a person.
But these same lines can also be spoken as very earnest and well-meaning. Paris, after all, has no reason to believe that Juliet doesn't love him and isn't excited about their marriage. This is the impression that he gets from Juliet's parents, after all. In this interpretation, Paris's confidence in their marriage comes across as pathetic rather than obnoxious. Either way, though, the "holy kiss" that Paris gives Juliet at the end of the scene is painful for everybody. It's so formal and stiff – a complete contrast to Romeo's kisses.