Romeo and Juliet both express love for each other, but what are Juliet's doubts and what does she say to express her concerns?
I assume that you are referring to Act II, Scene ii, which is commonly known as the "Balcony Scene."
Juliet expresses several concerns throughout the conversation she has with Romeo in Act II, Scene ii. Her first fear is that her family will kill Romeo if he is found in the Capulet orchard, but Romeo dismisses her worry. Juliet also reveals her anxiety at not adhering to typical rules of convention, such as having been "too quickly won"; she fears that Romeo will think that she is not being serious or is not modest. Basically, Juliet is afraid that she should have been more aloof and played "hard to get."
The most troubling of Juliet's worries is her belief that she and Romeo should not rush into things. Romeo panics when Juliet tells him of her fear, and she responds by reiterating that she does, indeed, love him.