Then they're probably laughing just to be part of the crowd and not stick out as a non-participant. You don't want to be the only person sitting dead silent while others are howling around you. You'd get comments similar to, "what's wrong with you?" or "what's your problem?"
The material...
I'm not sure that's it for me, as I love Jack Lemmon in the Grumpy Old Men films, Odd Couple 2, and My Fellow Americans (which is one of my favorite comedies).
I'll have to give this another try sometime. I know I've watched it at least once, but did not find it as hilarious as everyone claims it is. Not sure if that's a case of the film being overhyped for me as the greatest comedy ever made before I actually watched it, or if the humor just doesn't...