The primary difference between loving Rosaline and loving Juliet is that his love for Rosaline is unrequited. He spends the first several scenes of the play in various states of depression over the fact that he is "out of her favor" (I.i.168) though he loves Rosaline still. In contrast, Juliet loves him so much that she is willing to give up her Capulet name on the night they meet, vowing that her feelings are genuine. She begs Romeo to profess the same feelings toward her:
But trust me, gentleman, I’ll prove more true
Than those that have more coying to be strange.
I should have been more strange, I must confess,
But that thou overheard’st, ere I was ‘ware,
My true love’s passion. (II.ii.104-108)
In these lines, she effectively contrasts her feelings to those of the emotionally unreachable Rosaline.
It can also be argued that Romeo's love for Rosaline is childish. Proclaiming love for a person who has never shown similar feelings is not true love. True love is instead born out of shared experiences, both the peace shared in happy circumstances and the grief shared in life's tragedies. Rosaline shares none of this with Romeo. Thus, he doesn't truly love her; he is infatuated with the idea of loving her. Juliet, by contrast, is willing fake her own death and leave her family in order to be with Romeo. In the end, she is willing to die when she discovers that Romeo has committed suicide. Her emotions are deep and genuine, and Romeo's feelings reflect this depth of emotion. Their love is quickly lived, but there is a depth to the emotions between them that does not exist with Rosaline.