Romeo is instantly in love with Juliet as soon as he sees her, but figuring out why isn’t easy. They don’t really get to know each other at all; they are instantly, desperately in love. There is no sense of building interest or attraction; they simply are unaware of each other’s existence and then, suddenly, willing to die for each other.
How credible this seems to you will depend on whether you think “love at first sight” is truly possible. Some people swear they’ve fallen in love almost instantaneously, others insist that it’s impossible to truly love someone you barely know. Attraction works mysteriously, but don’t look for Shakespeare to explain too much about it: he’s okay with portraying love as something that strikes like a bolt of lightning, irrationally, and doesn’t spend time articulating why.