Romeo is the only son of Montague, sixteen-or-so, raised in money, raised in a feud he didn’t pick. We meet him already heartbroken — but not over Juliet. He’s mooning over a girl named Rosaline who has sworn off love, and he’s enjoying being miserable about it. He writes bookish, second-hand poetry. His friends tease him for it. His mother worries. The play starts with Romeo as a boy in love with the idea of being in love.
What Romeo wants changes in the time it takes to cross a ballroom. From “I’d like Rosaline to notice me” he goes, in one scene, to “I’d marry this girl tomorrow.” For the next four acts what he wants is simple and impossible: to be near Juliet. He wants the feud to not matter. He wants the world to leave them alone for one quiet night. The whole tragedy is built on how little of that he gets, and how willing he is to throw everything away to keep what he has of her.
Romeo grows up in three days. The boy in Act 1 quoting fashionable love-sonnets becomes the young man in Act 2 inventing better lines because Juliet is real. After Mercutio dies in his arms he picks up a sword for the first time and kills Tybalt — and the play stops being a romantic comedy. By Act 5, when he hears Juliet is dead, he doesn’t deliberate. He buys poison and rides to the tomb. The same impulsiveness that made him fall in love at a glance kills him at the end. Shakespeare doesn’t simplify it: Romeo is brave and reckless, tender and dangerous, and the play doesn’t sort those into separate boxes.