What is the reason Rosaline will have nothing to do with Romeo?
As a teacher, I always require my students to be able to defend what they think with evidence from the text. Her is exactly the discussion between Romeo and Benvolio:
Well, in that hit you miss: she'll not be hit
With Cupid's arrow; she hath Dian's wit;
And, in strong proof of chastity well arm'd,
From love's weak childish bow she lives unharm'd.
She will not stay the siege of loving terms,
Nor bide the encounter of assailing eyes,
Nor ope her lap to saint-seducing gold:
O, she is rich in beauty, only poor,
That when she dies with beauty dies her store.
Then she hath sworn that she will still live chaste?
She hath, and in that sparing makes huge waste,
For beauty starved with her severity
Cuts beauty off from all posterity.
We can only know for sure what Rosaline has said to him and what he then reports to Benvolio. Think about how teenagers operate too. The story becomes more dramatic from teen to teen for gossip's sake. Romeo's explanation grows whether he's just that infatuated, or whether he's defending his inability to convince her, or whether he's a gossip. The repetition of ideas makes me think he's infatuated. Also, when it comes to issues of sex (and chastity is an issue of sex being the choice not to get involved) Shakespeare loves to color it with language. So whether it's Romeo's character, Rosaline's lie and rejection or Shakespeare's craft we'll never know because we can't ask.
Take some time to think about the lines above and what they might be saying.
We have no way of truly knowing why Rosaline does not love Romeo. All we can know about her is what we hear from other characters -- ones who actually get to speak for themselves.
As far as we know, Rosaline does not want anything to do with Romeo because she has sworn to live in chastity. So that means it is nothing to do with Romeo -- she just does not want a man.
Now, of course, you can be suspicious of this. It sounds to me like something she might just be saying so she can get rid of Romeo. After all, he seems pretty immature and sulky in Act I.