What's wrong with smiling ??????

"What are you smiling at ?" Mum asks.

"Huh ?" was my reply, everytime she asked that.

"Your brain got problem isit ? Smiling like that at yourself, only people with mental problems will do that."

What's wrong with smiling when there's a nice and funny thought that's running in your mind ? What's wrong with it ??? I don't get it !!!~~~