Romeo is the typical Romeo: impulsive, ready to do anything for love. In one instant at the party, he falls madly in love with Juliet and forgets all about Rosaline. His rash action in visiting Juliet that very night (the "balcony scene" ) as well as his ready acceptance of Friar Lawrence's plan makes it clear that his heart rules his head. His fight with Tybalt is another example of his hot-hotheadedness One could almost say that he is childish and has little control over his emotions.
By contrast Prince Paris is a proper gentleman, who follows all the etiquette. He obeys his parents wishes and agrees to marry Juliet. He usually is in full control over his emotions and rarely acts stupidly or impulsively.