Why do men act boring and stern when around their wives but act full of life and playful when around their mistress?

Not me, I never had a mistress and don't want one. I am always happy and playful around my wife of 26 years and we worked together for 13 before that. Maybe that is why she married me and stayed..................or it is because I do most of the cooking and cook well, clean house and do the laundry.