In Romeo and Juliet, why does Mercutio make fun of Romeo after the Capulet party?

Mercutio csalls for Romeo using some bawdy language:

Nay, I’ll conjure too!
"Romeo! Humours, madman, passion, lover!
Appear thou in the likeness of a sigh!
Speak but one rhyme, and I am satisfied....By her fine foot, straight leg, and quivering thigh...act 2 sc 1