In the first scene, we hear from Benvolio that Romeo has been hiding in the woods, avoiding his friends. We learn from Romeo's father that Romeo has been out at night, crying and sighing, only to come home again at dawn to lock himself in his room in "artificial night." His father observes that he is sad all the time, and Benvolio vows to get to the bottom of the problem and find out what is wrong.
He discovers that Romeo is in love with Rosaline. We further find out that Romeo is pining because although beautiful, Rosaline has sworn to remain chaste, leading us to wonder if Romeo is more in lust than in love. Benvolio tells Romeo to forget about her. Romeo asks how he can possibly do that. Benvolio tells him to consider other women ("examine other beauties"). Romeo says that won't work ("thou canst not teach me to forget").
Romeo's behavior at this point could be described as histrionic and impulsive. He throws himself wholeheartedly into the mood of the moment and can't imagine he could ever feel any other way. He's given to excess rather than moderation in his emotions, which are all consuming. He seems to be a person of leisure, with no demands on his time that might distract him from lovesick pining.
By the end of Act I, Romeo has done a complete flip flop and fallen head-over-heels in love with Juliet.