To understand Romeo's reaction when he first spots Juliet, it is important to understand his state of mind before this scene. Up to this point, he was moping about Rosaline, the woman he claimed to love and who did not love him back. In Act I, Scene I, Benvolio finds Romeo moping. He explains that he is in love and sad about it. Benvolio's response is to tell Romeo to get over it by finding another woman:
By giving liberty unto thine eyes/Examine other beauties (218-219).
Romeo expains that he couldn't possibly love another woman. This is why his reaction to seeing Juliet at the party that night is so surprising. He seems to completely forget that he was "in love" just that day. His reaction is immediate and intense as he compares her to "a rich jewel in an Ethiope's ear" (44), meaning her beauty stands out like a jewel against black skin. Most surprisingly, he goes on to say
Did my heart love till now? Forswear it, sight!/ For I ne'ever saw true beauty till this night" (50-51).
This completely contradicts everything he said earlier about Rosaline. He swore he would never see anyone more beautiful than Rosaline, but is able to fall out of love with Rosaline and in love with Juliet in a moment. It is this attitude toward young love that contributes to the downfall of the two young lovers in the end of the play.