In Act II, Scene 4, the nurse tells Romeo that Juliet is the sweetest lady. She doesn't want Romeo to get Juliet's hopes up to let her down because she is young and innocent and as most 13 year olds are, impressionable. She wants Romeo to be honorable and true to his word.
Act II, scene iv, 103-110
Well, sir, my mistress is the sweetest lady.—Lord, Lord! when 'twas a little prating thing.—Oh, there is a nobleman in town, one Paris, that would fain lay knife aboard, but she, good soul, had as lief see a toad, a very toad, as see him. I anger her sometimes and tell her that Paris is the properer man. But, I'll warrant you, when I say so, she looks as pale as any clout in the versal world. Doth not rosemary and Romeo begin both with a letter?
Basically, she is saying this:
Well, sir, my mistress is the sweetest lady. Lord, Lord, when she was a little baby—Oh, there is one nobleman in the city, a guy named Paris, who would be happy to claim her as his own. Juliet would rather look at a toad than at him. I make her angry sometimes by saying that Paris is more handsome than you are. But when I say so, I swear she turns white as a sheet. Don't “rosemary ” and “Romeo” begin with the same letter?
The nurse describes Juliet, in Act 2 scene 5, as young and sweet. Much of her speech seems to be about Juliet's need for protection and honest dealing. She tells Romeo it would be "gross behaviour" if he were to lead her into a "fool's paradise" because of her age--the implication being that she is vulnerable and, perhaps, almost too innocent for what she is about to do. At the same time, she makes sure Romeo knows he has competition, and not just competition for her love, but sexual competition as well(she says there is another suitor who would "fain lay knife aboard", another triple play on word meaning, all of which suggest various degrees of claim staking and ownership, with the final allusion being more graphic but no less territorial). So she says Juliet is young and innocent and has the potential to be easily hurt, yet in almost the same breath she suggets that Romeo is not the only one who wants her.