First of all, she knows he is from an enemy family, and that their...
Juliet is concerned about several things before, during, and after her conversation with Romeo, and for this reason, she is not completely happy with the promises of love they make in Act II, scene ii.
First of all, she knows he is from an enemy family, and that their love will not be accepted by their families. She is musing on this when Romeo first overhears her - she is thinking about the fact that it is really just their names that are enemies. Here she asks the famous question, "What's in a name? That which we call a rose by any other name would smell as sweet."
Romeo then overhears her proclaiming her love for him before she is aware he is in the garden, so she is concerned that he will find her too forward and perhaps take advantage of the fact he already knows her feelings for him. Before she knew he was there, she had even gone so far as to urge Romeo:
Deny thy father and refuse thy name;
Or, if thou wilt not, be but sworn my love,
And I'll no longer be a Capulet.
Romeo, doff thy name
And for that name which is no part of thee
Take all myself.
When Romeo responds to these last words, Juliet is taken aback at his presence. She tells him how embarrassed she is knowing he overheard her innermost thoughts:
Thou know'st the mask of night is on my face,
Else would a maiden blush bepaint my cheek
For that which thou hast heard me speak to-night.
She goes on, though, to ask if he loves her in return. Even his positive response, however, she admits will not make her happy, for she knows he may be telling a lie:
I know thou wilt say 'Ay,'
And I will take thy word: yet if thou swear'st,
Thou mayst prove false; at lovers' perjuries
Then say, Jove laughs.
When Romeo goes on to "swear by the moon" that he loves her, she quickly chastises:
O, swear not by the moon, the inconstant moon,
That monthly changes in her circled orb,
'Lest that thy love prove likewise variable.
After she finally takes Romeo at his word, she agrees to marry him the next day if he is able to make the arrangements for the wedding. However, she does not truly feel at peace with this decision, as shown when she states:
I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning which doth cease to be
Ere one can say it lightens.
The friar echoes this sentiment a couple of scenes later when he warns that "these violent delights have violent ends" and urges both of them to "love moderately: long love doth so."