We are not actually given any reasons by Shakespeare to dislike Paris or to think that he is an unworthy match for Juliet. On the contrary, the characters throughout the play actually speak very highly of him. Also, in every scene in which we see him, he portrays himself as a kindly gentleman. Not only that, his affection for Juliet is portrayed as very sincere. He is wealthy, has a high social station, and will be able to provide well for Juliet. He is also presented as being far more mature and sophisticated than Romeo. His social status and maturity coupled with the fact that he is kind and sincere should actually make us question whether or not Juliet made the right choice in Romeo.
We first see Paris being spoken highly of in the third scene when Juliet's mother first proposes the idea to Juliet of marrying Paris. Juliet's mother refers to Paris as "valiant" in line 78 and Nurse echos her sentiment, declaring him to be "a man, young lady! lady, such a man / As all the world." Nurse further characterizes Paris as a "man of wax," meaning a very attractive man. In fact, Nurse is very excited about the prospect of Juliet marrying Paris, even prophesying Juliet's future happiness in the scene's closing line, "Go, girl, seek happy nights to happy days," meaning, let these upcoming happy nights bring you happy days, referring to the upcoming ball and her upcoming wedding night.
Not only does Nurse make it evident she thinks Paris is a lovely prospect, she outright criticizes Juliet's choice in Romeo, saying, "Well, you have made a simple choice; you know not / how to choose a man. Romeo? No, not he;" but then goes on to tease Juliet about Romeo's good looks (Act 2, Scene 5). However, Nurse's preference for Paris is once again reiterated when Juliet's wedding date is moved up to Thursday. Nurse advises:
I think it best you married with the county.
O, he's a lovely gentleman!
Romeo's a dishclout to him: an eagle, madam,
Hath not so green, so quick, so far an eye
As Paris hath.
Juliet's reasons for choosing Romeo were purely naive. He was the very first handsome man to kiss her, therefore she allowed herself to be governed by her hormones, quickly thinking that she was in love with him, though she knew very little about him. In contrast to Paris, we see that Romeo is prone to being very emotional, even irrationally so, as we see with his reaction to being rejected by Rosaline. He is also stubborn and refuses to be counseled by others. While Romeo is older than Juliet, mentally and emotionally he is still extremely immature. Since Romeo is immature while Paris is not, it seems evident that Paris would have made a better husband for Juliet than Romeo. Therefore, it seems Juliet's choice in Romeo was rash and naive.