One passage that portrays Romeo's immaturity can be found in the very first scene. When it is clear that Romeo is letting his heart be eaten away by his emotions for Rosaline, Benvolio begs him to listen to his advice and to forget about Rosaline. Romeo's response is, "O, teach me how I should forget to think!" (I.i.227). This line shows emotional immaturity due to the fact that it also portrays Romeo as being irrational. Only an irrational, immature person believes that he/she has no control over his/her own emotions. A wiser man, like Benvolio, would see that we can use our rational minds to control our emotional responses to situations.
Impulsiveness can be seen in Romeo's decision to marry so suddenly, particularly when he begs Friar Laurence to marry he and Juliet that day, as we see in the line, "But this I pray, / That thou consent to marry us to-day" (II.iii.64-65). There was no real, concrete reason to want to be married so suddenly. Instead, had they postponed the marriage, Friar Laurence might have acted as an intermediary to try and persuade both Lords Capulet and Montague to consent to the marriage. Delaying the marriage until their plans were more publicly known and publicly accepted might have saved both Romeo's and Juliet's lives.
I have always thought that the string of oxymorons Romeo uses to describe how lovesick he is, is a bit dramatic and shows that he is impulsive about his love for Rosaline.
"Alas, that love, whose view is muffled still,