Because she finds it funny that Mitch is so innocent, and he told her his feelings without any shame.
Would you sleep to me tonight? Oh, you don't understand me? What a shame...
It is related to their social status. Immigration was important in this region, and people from French ascendence.
It's something he's very proud about. He believes that he's not that smart, so he talks about his body in order to impress Blanche.
Because he really loves her and is willing to go further than just an affair.
It represents love. After her husband died she was hopeless, lost and didn't know what to do.
It represents her "past", roaring past her and tormenting her.
Because she got mixed up with a 17 year old student, at her own school.