Romeo's behavior might be considered quite immature and reveals an individual who has probably been given everything he has ever wanted, making him incapable of accepting rejection when a woman he claims to love does not return his affection. Romeo is also mercurial in his behavior in Act I of Romeo and Juliet as he abruptly abandons his love for Rosaline in favor of Juliet.
Before Romeo even enters the scene his behavior is described as immature. He is acting moody and often goes into his room in the middle of the day and shuts himself up in darkness. His parents are worried about him but cannot find out what is bothering him and so they turn to Romeo's cousin, Benvolio, to discover Romeo's problem. When the audience meets Romeo they find a young man who is tormented by the idea that Rosaline has refused his advances and has "sworn" to "live chaste." Judging by his rant to Benvolio about Rosaline, it is obvious that Romeo has rarely faced hardship or rejection in his life. As the only son of wealthy Montague he displays the traits of a spoiled and pampered child. He is unable to believe or accept that Rosaline could choose not to love him.
Romeo's changeability is highlighted later in Act I when he does a total about-face in his romantic feelings by totally forgetting Rosaline "and that name's woe" after seeing Juliet at Capulets' party. He is instantly mesmerized by the girl, possibly in the same way he was mesmerized by Rosaline. The difference, of course, is that, unlike Rosaline, Juliet returns Romeo's adoration and is equally in love. It would be interesting to know what Romeo would have done if Juliet had rejected him. In any event, Friar Laurence's words in Act II, Scene 3 might be the best summation of Romeo's attitudes and behaviors:
Young men’s love then lies
Not truly in their hearts, but in their eyes.
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.
The first scene of Act 1 reveals how distraught Romeo is that the girl that he loves – Rosaline – is not in love with him. Lord Montague first makes mention of this when he asks Benvolio to find out what is the matter with Romeo because he locking himself in his room, sitting alone in the dark, and has become very depressed lately. Upon asking Romeo what his problem is, Benvolio finds out that Romeo is in love with Rosaline but she tells him that she does not love him back. By the end of Act 1, Romeo has met Juliet and has completely forgotten about Rosaline. By this point, Romeo is no longer upset or depressed and has a much more positive attitude and outlook because he knows that Juliet feels the same about him as he feels about her.