Why is Juliet embarrassed? 

The reason that  I think Juliet was embarrassed was because she was truly saying how she felt about Romeo but didn't necessarily want Romeo to know and so when Juliet realizes that Romeo had heard everything she said she was embarrassed.  As Shakespeare said: "Thou knowest the mask of night is on my face, else would a maiden blush bepaint my cheek for that which thou hast heard me speak tonight"(Shakespeare,75). This shows that Juliet was so embarrassed that she was even blushing.