"Why?" is sooo over already

"Why?" is sooo over already

Apparently, DD got wind of the news that asking "Why?" is just not fashionable. It is sooo last season. So, she's decided on a new question to ask to everything I tell her.

"Why not?"

"Don't put that in your mouth"
"Why not?"

"Eat your dinner."
"Why not?"

"Wash your hands please."
"Why not?"

"I'm going to do the laundry"
"Why not?"

"I don't want to have to tell you again."
"Why not?"

Ok, so you get the picture. I seriously don't know where she picked it up from. She just suddenly switched from "Why?" to "Why not?"