Romeo and Juliet question

Mmm, I think that Romeo believes that Rosaline is the ideal of what he should love. Maybe like being infatuated with a super model, or similar unattainable figure.
Juliet is a real girl - and she returns his affections (which is important)
Maybe that is her purpose - showing us the difference between what love really is (who it is and how it grows) vs what we think it is before it actually happens.
Just a thought