Why is Juliet upset at the end of act one and why romeo beginning one?

At the end of Act 1, Juliet is upset because she has fallen in love with Romeo, a Montague, who is the son of her family's sworn enemy. This love creates great turmoil within her as she grapples with loyalty to her family's feud and newfound affections toward Romeo.

At the beginning of Act 1, Romeo is depressed because he is in love with Rosaline, who does not return his feelings. He feels rejected and heartbroken and expresses his grief and despair through soliloquies and interactions with friends like Benvolio.