Ironically, Romeo's love for Juliet seems very similar to his love for Rosaline. What little we know of his relationship with Rosaline comes from Act 1 Scene 1, where Romeo laments that his love for Rosaline is not reciprocated because she wishes to remain chaste and become a nun. In this scene with his cousin, he goes on about how she is wasting her beauty.
She hath, and in that sparing makes huge waste;(220)
For beauty, starv'd with her severity,
In this very short description of Romeo's broken heart, the reader must make a few inferences. First, it is unlikely that he had a relationship with Rosaline at all. It does not sound like she broke up with him, but rather that she was never interested in him. In this way, his "love" for her is but a distant attraction, and most likely, this is mostly to her looks. Romeo is young and sensitive and overly emotional. It is speculated that this scene reveals his fickle and immature personality when it comes to "love."
Therefore, when Romeo "falls in love" with Juliet at the party, it is almost identical to his love for Rosaline. That is to say, Romeo is physically attracted to Juliet, and equates this attraction with "love." He is equally hasty to fall for Juliet and immediately forgets about Rosaline. Though it might be assumed that Romeo and Juliet's relationship is more "real" or actually true love, the audience never knows, because the two meet, marry, and die within the context of a few days.
Romeo to Rosaline's love is fake and will eventually fade away and Rosaline doesnt love him back. Romeos love for Juliet will be long lasting, and Juliet loves him back.