In act 1 of Romeo and Juliet, Romeo reveals himself to be melancholy, self-centered, and highly changeable. As the play opens, Romeo is in love with a woman named Rosaline who has refused his love. Therefore, Romeo is moping and miserable. “Griefs of mine own lie heavy in my breast,” he complains. “I have lost myself,” he continues, adding, “I am not here; / This is not Romeo, he's some other where.” Rosaline has scorned him. She is “too fair, too wise, wisely too fair,” Romeo explains, and she has sworn not to love anyone. Therefore, he laments, “in that vow / Do I live dead that live to tell it now.”

Indeed, Romeo is far more caught up in his own feelings than he is in Rosaline. He is focused on himself even as he thinks he is focused on her. He is focused on how wretched he feels. “O, teach me how I should forget to think,” he exclaims to Benvolio, who wisely tells him to look at a few other women. Romeo doesn't think that will work, for other women will only remind him of how beautiful Rosaline is. Actually, Romeo doesn't really want help. In fact, he seems to rather enjoy his suffering.

Yet when Romeo catches sight of Juliet, he forgets all about Rosaline in an instant (so much for being truly in love!). “O, she doth teach the torches to burn bright!” he exclaims. Then he asks himself, “Did my heart love till now? forswear it, sight! / For I ne'er saw true beauty till this night.” At this point, Romeo doesn't even know who Juliet is. He just sees her and instantly loves her. We wonder, of course, if Romeo has any idea what love actually is since he can drop one woman (the woman he claims to love to the point to obsession) and quickly become enraptured with another. Indeed, he is changeable and quite shallow. He thinks he feels deeply, but his emotions are primarily on the surface, and they will lead to tragedy.

In Act I of Romeo and Juliet, Romeo is characterized as spoiled, introspective, and impetuous. As the son of the wealthy Montague family, Romeo is the product of privilege. When his love is spurned by a woman named Rosaline, he sulks and withdraws from society. He is a spoiled child who has had very little experience with rejection. He expresses this somewhat infantile behavior in his rant to Benvolio in Act I, Scene 1. He is outraged Rosaline refuses his advances and claims she refuses to be in love:

She’ll not be hit
With Cupid’s arrow. She hath Dian’s wit,
And, in strong proof of chastity well armed,
From love’s weak childish bow she lives uncharmed.
She will not stay the siege of loving terms,
Nor bide th’ encounter of assailing eyes,
Nor ope her lap to saint-seducing gold.
O, she is rich in beauty, only poor
That, when she dies, with beauty dies her store.

You have dancing shoes
With nimble soles. I have a soul of lead
So stakes me to the ground I cannot move.

I fear too early, for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date
With this night’s revels, and expire the term
Of a despisèd life closed in my breast
By some vile forfeit of untimely death.

In Act I Romeo mostly shows his "lovesick" quality.  That defines him most of Act I.  When speaking of Rosaline's beauty and comparing her to others, he says to Benvolio,

"One fairer than my love! the all-seeing sun
Ne'er saw her match since first the world begun."

Then on his way to the Capulet party in scene 4, he tells the guys that he will go, but his heart is too heavy for him to dance and have a good time.

"you have dancing shoes
With nimble soles: I have a soul of lead
So stakes me to the ground I cannot move."

Then in scene 5 we see Romeo's tragic flaw.  He shows how rash he can be.  As soon as he sees Juliet, he says,

"Did my heart love till now? forswear it, sight!
For I ne'er saw true beauty till this night."