What about Romeo made Juliet fall in love with him?
Juliet is about to answer this question, I think, when Romeo interrupts her at her window (the play says nothing about balconies!) and she changes tack.
Why do people fall in love? It must be looks, I think, to some extent, but also an attraction of personality - the latter is neatly and subtly underlined by Shakespeare in the way that Romeo and Juliet's first lines in the play form a perfect sonnet, the traditional form of romantic love.
To get back to Juliet's answer, the first thing she says is that she doesn't like the fact that Romeo is a Montague, but that she does like the fact that he is "himself" - and then itemises body parts that she likes:
Thou art thyself, though not a Montague.
What's Montague? it is nor hand, nor foot,
Nor arm, nor face, nor any other part
Belonging to a man.
Moreover, she says, if Romeo was not called Romeo, he would retain his "dear perfection":
So Romeo would, were he not Romeo call'd,
Retain that dear perfection which he owes
Without that title. Romeo, doff thy name;
And for that name, which is no part of thee,
Take all myself.
Why does Juliet love Romeo? Because to her, he is dearly perfect, simply because he his himself. That's love, for you, I suppose!