The Romeo and Juliet story, is actually based on an earlier Italian story. The origin of the story are uncertain though it was mashed up a number of times before Shakespeare. Some ascribe it originally to Masuccio Salernitano, who wrote a similar story around 1476. According to Harold Bloom, Shakespeare was not that original of writer and he took his story lines from here and there – today, we call this “plagiarism” – in the past, it was called creativity. Shakespeare’s genius was in his characterizations and in his command of language.
Love at first sight is part of the Petrarchan paradigm of unrequited, or pure love. The whole “tale of woe” unfolds over three days, from first sight to tragic death, so if the Monague’s and Capulets blinked, they might have missed it.
The story obviously strikes a familiar chord with a lot of people. My favorite remake is West Side Story.