Romeo's behavior changes significantly after he meets Juliet at the Capulet estate. How does he act differently after their encounter, and what conclusions can be made about Romeo because of this change?
Before Romeo sees Juliet dancing from across the room, he can think of no other woman than Rosaline. He has been moody for weeks; his parents have been worried about him as he broods in the darkness of his room. Even his close confidant and cousin Benvolio cannot shake Romeo out of his gloomy state. Rosaline has rejected him, and because of this he feels that he will never find love. Benvolio offers the idea of going to the party at the Capulet's house so Romeo can see other women and forget Rosaline, "making (his) swan a crow." Romeo agrees to go only so that he may see Rosaline.
But Romeo's mood changes very quickly when he sees Juliet. He immediately forgets Rosaline and falls in love with Juliet. As they have their first kiss, Romeo aggressively pursues Juliet who is at first resistant to his forward approach.
This change in mood is the crux of the play. Some like to call Romeo and Juliet a love story of epic proportions. But I think Shakespeare was up to a little bit more than that. Throughout the play, the characters act on their emotional instincts instead of their reasoning. It leads to blunder after blunder, and eventually Romeo and Juliet's premature deaths. Romeo is the most prone to these huge emotional swings, falling in love with Juliet after about five minutes, taking revenge on Tybalt for the death of Mercutio, and weeping like a small child when he is banished. Through this first exchange, we learn that Romeo is mercurial at best, violently moody at worst.