I'm not going to write your whole answer for you, but here are some points to think about:
The balcony is outside of Juliet's bedroom window. This is an intimate place to be. In fact, when she first starts talking she thinks she's is alone, and does not realize Romeo is listening to her talk about him.
If Romeo is caught on Juliet's property he will be, at best, beaten up by her relatives and, at worst, killed by them. This gives a sense of fear and urgency, but also adds to the delicious secret.