In Act I, Scene 1, when we see Romeo speaking with his cousin, Benvolio, Shakespeare fills Romeo's part of the dialogue with oxymorons and paradoxes. This makes Romeo seem rather dramatic, and it makes his love for Rosaline feel, perhaps, like it is not love, but rather infatuation or lust. He says,
Why then, O brawling love, O loving hate,
O anything of nothing first created!
O heavy lightness, serious vanity,
Misshapen chaos of well-seeming forms!
Feather of lead, bright smoke, cold fire, sick health,
Still-waking sleep, that is not what it is! (Act I, Scene 1, lines 166-171)
Romeo gets quite carried away with his riddles, and his dramatic sadness makes him seem a little immature. He claims his unrequited love for Rosaline renders him totally miserable.
Further, Romeo uses allusions when he discusses Rosaline's relationship to figures in Roman mythology:
She’ll not be hit
With Cupid’s arrow. She hath Dian’s wit.
And, in strong proof of chastity well armed
From love’s weak childish bow, she lives uncharmed (Act I, Scene 1, lines 199-203).
Romeo claims Rosaline is immune to Cupid's arrows; it's as though she cannot fall in love. Further, he compares her to Diana—a virgin goddess who protects youth—especially in terms of her chastity and unwillingness to sleep with him. These allusions do make him seem intelligent, or at least educated. Romeo seems somewhat quick and witty, then, if a bit immature.