Because love is impossible for them.
Because Romeo is handsome and Juliet is beautiful.Because they are same to each other like an object and its image in the mirror.
Because they want to make their families angry.
Because Friar Lawrence wants them to fall in love with each other.
Because Romeo is Casanova.
Because the party is cool and they are drunks.
Because they are teenagers.
Because they want to have sex with someone.
Because Juliet is virgin.
Because of their outside looking.
Because the relationship between their families are not good enough. And they want to fix it.
Because they are crazy people.
Because their lives is ordinary and boring and they need something different.
Because of the smell of their skin.
Because Romeo wants to forget his ex-girlfriend.
Because Eros shoot them with an arrow of love.
Because they need to love someone without any reason.
Because their love is not in their heart. It is in their eyes.
Because of their hormones.
Because they are Romeo and Juliet.