She thinks he's unfunny and often points out the lameness of his humour. On top of that, she laughs with the annoying "You're getting better" comment.
But there's the catch.
He thinks he has always been funny and she is merely getting entertained without giving credit where it's dude. Granted that he cracks some of the lousiest original jokes in the history of stupidity but that doesn't mean he can't make her laugh. For the insider record, her laughter lasts longer than her orgasms. Yet, she won't admit that he's genuinely funny. Maybe it's a feminine thing to do. Like women keep going to washroom at a party (but not in office as they deliberately don't consume enough water during working hours) without anybody really knowing why.
Maybe the catch here is she goes to loo repeatedly even in their house so as to laugh out loud. After all, it's not humanly possible to suppress one's laughs from his supremely hilarious quips for long.