As act 2, scene 2 opens, Juliet voices concern about Romeo's identity as a Montague (the family that has been caught up in a feud with her own). That she has fallen in love with a member of this rival family seems to have caused her a great deal of distress. These anxieties (and her awareness of the practical difficulties connected with the feud) are reflected in her reaction when Romeo announces his presence to her.

When she discovers Romeo, she asks him how he had gotten into the orchard and voices concern about the consequences should he be found. She knows that, as a Montague, he is viewed by the Capulets as an enemy, and if he were to be discovered in this context, she is certain he would be killed (though Romeo does not seem to take this danger as seriously as Juliet herself).

At the same time, Juliet also voices concerns about Romeo's declarations of love for her (and the mindset lying behind them). Even if she accepts and returns that love, she is aware that this kind of attitude can be capricious or dishonest, and she asks for a more meaningful declaration of his intentions. Thus, Juliet brings up the subject of marriage. She states that if his intentions are dishonorable, she'd prefer if he ceased pursuing her altogether.

First, Juliet has fallen in love with Romeo, and his arrival at her residence as a not-quite fourteen-year-old chaste girl is reason for concern:

The orchard walls are high and hard to climb,
And the place death, considering who thou art,
If any of my kinsmen find thee here. (II.ii.67–69)

A young woman's honor was highly valued in this society, and Juliet's father (and kinsmen) would have taken great lengths to preserve every image of virginal integrity. Romeo's uninvited and unchaperoned appearance is a risky endeavor, and Juliet is understandably worried about his well-being. This is compounded by the fact that Romeo's family is the enemy of the Capulets.

Juliet is also a bit worried that Romeo's intentions toward her are less than noble:

Dost thou love me? I know thou wilt say “ay,”
And I will take thy word. Yet if thou swear’st
Thou mayst prove false. (II.ii.94–96)

Juliet is young and inexperienced in the ways of love, and she realizes that Romeo holds the upper hand in knowledge of romance. She isn't sure whether to play things cool and aloof or sincere and genuine. She tells Romeo that she believes that she likes him "too much" and that perhaps she is "too quickly won." Her youth makes her second guess herself and how she should behave under these circumstances. Juliet is worried that Romeo isn't as serious as she is about their new relationship, and she begs him to swear his true love to her so that she can know of his real intentions.

Thus, Juliet is worried because of her status as a chaste young girl, because her family hates the Montagues, and because she isn't quite sure how to navigate the complex waters of romantic love as this is seemingly her first venture into love of this depth.

There are a couple of things that are concerning Juliet by the time she realizes that he is present in the garden in Act II, Scene II.

Juliet's first concern is that, should her family see him their, they'll kill him. She sees that Romeo has accomplished a fantastical feat by daring to enter the garden. The garden walls have especially been built to keep out enemies, yet Romeo has managed to climb the high garden walls that are difficult to climb. But more importantly, Romeo has accomplished this feat for no known reward other than certain death should he be discovered, as we see in her lines:

The orchard walls are high and hard to climb,
And the place death, considering who thou art,
If any of my kinsmen find thee here. (II.ii.67-68)

Juliet's second concern is that she is embarrassed at being overheard by Romeo in professing her love for him. She's a maiden, and in those times virtuous maidens were expected to be less forward and much less easily won. We see Juliet express her maidenly embarrassment in the lines:

Thou knowest the mask of night is on my face;
Else would a maiden blush bepaint my cheek
For that which thou has heard me speak to-night. (89-91)

She further alludes to her role as a respectable maiden by saying that if he thinks she has been too easily courted, she'll pretend that she does not care for him. Easiness or flirtatiousness in a woman can be a sign of sexual looseness; therefore, Juliet is very concerned about keeping her reputation in tact, as we see when she says, "[I]f thou thinkest I am too quickly won, / I'll frown, and be perverse, and say thee nay" (99-100). To be "perverse" means to be "stubborn" or "difficult"; therefore, in these lines, Juliet is saying that she'll refuse him just so he can pursue her, just as a man ought to pursue a virtuous maiden.