One of Romeo's weaknesses, or character flaws is that, like a lot of youths, he is extremely stubborn. He is the type that refuses to listen to advice. We see this of him in the opening scene when Benvolio pleads with Romeo, "Be rul'd by me: forget to think of her," meaning Rosaline, which is very good advice considering that Romeo is making himself misserable by giving into his emotions and pining for her night after night (I.i.227). However, Romeo's stubborn reply is "O, teach me how I should forget to think," meaning that the only way he could not think of Rosaline is if he suddenly became incapable of thinking at all (228). Romeo's refusal to listen to his cousin's sound advice shows us just how stubborn Romeo is.
A second weakness of Romeo's is that he has a tendency to be overemotional instead of using his rational mind. Not only do we see this in his reaction to Rosaline, Friar Laurence chastises him for his overemotional reaction to his banishment. Friar Laurence refers to his "tears" as "womanish" and that his "wild acts denote / The unreasonable fury of a beast" (III.iii.116-117). In calling him "womanish," Friar Laurence is calling him soft-hearted and irrational. Likewise, in calling him a "beast," Friar Laurence is accusing him relying on his emotional instincts rather than his reasonable mind.
One strength of Romeo's is that he is known for his sincerity, especially his sincere affection. The fact that he pined over Rosaline night after night shows us that he sincerely believed himself to be in love with her and his love lasted quite a while. We know his love for Rosaline lasted a long time because Romeo reminds Friar Laurence that he "chid'st [Romeo] oft for loving Rosaline," meaning that he often scolded Romeo about his lust for Rosaline. We also know that Romeo's affection for Rosaline was believed to be sincere, because Friar Laurence is shocked by Romeo's sudden change of heart, declaring,
Holy Saint Francis! What a change is here!
Is Rosaline, that thou didst love so dear,
So soon forsaken? (66-68)
Friar Laurence's reference to Romeo's "love so dear" proves that Friar Laurence did believe, as did Romeo believe, that his love for Rosaline was sincere.