There is an electricity between Romeo and Juliet--"love is love" as the previous post so succinctly points out. Sometimes, there is a chemistry of sorts between two people. Of course, the fact that Romeo is on enemy territory already has charged his emotions, so he perhaps generates some of these charged ions in the direction of Juliet. "Vibes," as they are called.
There is reason to think that Romeo is as much in love with the idea of love as he is in love with Juliet. As pohnpei397 points out, he was in love before he even met the girl. He had been in love before also and this extreme of passion is part of his character, regardless of the love object.
We can't know for sure, but it sure seems like he just thinks that she is really beautiful (at least at first). Remember that he completely falls for her without ever speaking to her. It's love (or something) at first sight and looks are the only thing that can bring that on.
I totally agree with e-martin in #3. Romeo was on the rebound and Juliet was a pretty girl who showed some interest in him. Case closed. If he had met her under more emotionally normal circumstances he might not have even fell for her. Under normal circumstances he might have decided she wasn't worth the risk of her father's wrath.
Lesson: don't go to a party to meet girls so soon after a bad breakup. Stay home and mope instead. In time you'll get over it and you could end up saving a lot of people's lives that way : )
Because love is love.
No one can interpret the meaning of love.
"Heart of Romeo" loved Juliet.
When you love someone you can not ask yourself, why do you like this person!?
Because this is a difficult question and complex.
At the same time a question is easy...
But as said, "4":
Is love at first sight.