Why does mercutio make jokes about Rosaline

The primary reason Mercutio makes fun of Romeo is because he has a strong feeling Romeo is hiding nearby and means to lure him out by insulting him. Not having witnessed Romeo's dance with Juliet, Mercutio believes that nothing has changed in the last hour or so and that Romeo is still in love with Rosaline. Mercutio mocks Romeo's feelings for Rosaline, letting us know that not only does he think Romeo is a fool for falling in love, but he does not think falling in love is respectable.

Explanation:

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to show how the cables work.