Why funny ads work?

Laughter is a sign of approval
People relate with the joke.
Relating with the joke people relate with the product.
Then they buy the product because it represents / defines them.

Also
Because laughter is emotional.
An emotional person has difficulties distinguishing logical fallacies in the message.

Also laughter helps to convey messages otherwise too sensitive. But that's another story.

Yup, It's funny 'cos is true.