Following Sunday's masquerade, Romeo did not return home. Instead, he slipped into the Capulet orchard — in direct violation of approximately seventeen different laws regarding trespass and family feuds — and hid beneath Juliet's window.
What he overheard was Juliet, believing herself entirely alone, speaking his name to the night air and wondering — quite philosophically — why his family name should matter at all. Romeo, never one to let an opportunity pass, revealed himself.
"What's in a name? That which we call a rose by any other word would smell as sweet."
What followed, by all accounts from the Nurse who eventually came looking for Juliet, was several hours of vows, whispered promises, and at least one formal proposal of marriage. By dawn, they had agreed: they would marry in secret the very next day.