Romeo is an impulsive and changeable adolescent who is in love with love. If the play wasn't so filled with tragic events, we could almost laugh at Romeo over how quickly he moves from Rosaline to Juliet.
As the play opens, Romeo's father approaches Romeo's friend Benvolio, worried that Romeo is moping around all the time. Benvolio informs him that Romeo is in unrequited love with Rosaline and promises his father to try to take Romeo out and introduce him to other girls.
When Mercutio and Benvolio persuade him to go to the Capulet masquerade ball with them, Romeo considers it utterly pointless. He declares that there could never, ever, possibly be any woman for him other than Rosaline. However, he is hardly at party a few minutes when he sees Juliet, looking like a brilliant jewel that darkens everyone else in contrast. He then forgets Rosaline on the spot and instantly falls head over heels in love with Juliet. If your head is spinning, you are not alone: his good friend Mercutio is still ribbing him about Rosaline after Romeo has already made plans to marry Juliet.
Romeo lives with utter intensity in whatever moment he happens to be in, thinking that what he feels in that moment is what he will feel forever and ever. This behavior can be typical of adolesence, when young people are are grabbing life and holding it with both arms. Friar Laurence warns Romeo to calm down and stand back so that he doesn't burn out the love he has for Juliet, but that is not Romeo's way of approaching life.