We might first ask what on earth "lovesick callowness" is—a term we don't normally interject into every day conversation. Callowness is immaturity and inexperience. Lovesickness, in this context, is moping from unrequited love: it is being attracted almost obsessively to someone and that person not caring.
From the beginning of the play, Romeo's father expresses worry about Romeo's strange drooping about, questioning Benvolio about it. Benvolio explains that Romeo is suffering from love for Rosaline: he can't stop thinking about her, but she doesn't return his affection.
We realize that Romeo's lovesick pining for Rosaline is immature or callow when he immediately, on seeing Juliet, falls in love with her and forgets all about Rosaline. This happens so fast our head might be spinning. What happened to the girl who was so wonderful Romeo couldn't imagine possibly meeting anyone else who could rival her?
We might think at first that the main or only difference between Rosaline and Juliet...
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