In this scene, Romeo turns instantly. Romeo, who has up until this point been completely depressed that Rosaline will not go for him, lays eyes on Juliet for the first time and says:
Did my heart love till now? Forswear it, sight!
For I ne'er saw true beauty till this night.
He was saying these same sentiments about Rosaline a few moments before this. Therefore, love can come and go for him. He is a waverer, he changes, he is fickle.
It also demonstrates how much Romeo is in love with the idea of having a girl to love. And, he is much led by his eyes. He has no idea who Juliet is, yet believes he has fallen in love based on looks alone. She could have a terrible character for all he knows.