During the Capulet party, Romeo sees Juliet and immediately falls in love with her. (citation)
When Romeo is first introduced, he’s lovesick over Rosaline and emotional over the fact that she doesn’t love him back. His commitment to Rosaline quickly dissipates when he sees Juliet. The fact that Romeo’s deep and intense feelings change so quickly is evidence of his impulsiveness.
Without a care for Rosaline, Romeo defied the feud rules and went to see Juliet. (quote)