I guess I'm the only person who watched "Friends." (For those who don't know, Roger is a psychologist that Phoebe was dating during this episode). Just generally, Chandler likes to make people laugh - he tells jokes. It is one of his character traits. But when Chandler is uncomfortable or upset or unhappy or feeling anything bad, he can't stop telling jokes. When Chandler thinks that someone else is feeling anything bad, he tells jokes until they are laughing. It is his way of feeling a little bit better or at least pretending that he feels a little bit better. Along with that, if he couldn't make people laugh or if he couldn't tell jokes, he would feel really, really bad. That's what is meant by "when the laughter stops." Anyone who is around him will notice this, but the polite thing (in the US) is to just ignore it and laugh at his jokes. It is rude to say something like "I think you have a psychological problem" especially if you barely know the person (the way that Roger barely knows Chandler).

That's a tough one to answer without seeing the actual scene. It sounds like there's a lot going on there. My immediate interpretation is that Roger is being sarcastic and doesn't truly believe that Chandler is funny. It sounds to me like the line, "I wouldn't wanna be there when when the laughter stops," implies that there will be some kind of fight or negative consequence for Chandler when people people stop pretending to laugh at his jokes that aren't funny. You might have to wait for someone who's seen the show to come along before they can really answer your question though.

There is a belief that clowns, comedians, joke tellers are actually some of the saddest people in society, hiding pain (depression, trauma etc.) in humour. I'd believe (without watching it), that Roger is suggesting when the laughter stops the pain/hurt - maybe anger, will surface. Chandler is probably upset because of the implications that he is not happy, and maybe damaged psychologically. I've only seen a little of friends but Chandler did have a difficult time growing up, he has a difficult relationship with his parents. He is very self depreciating.

I haven't watched this series. Perhaps Chandler thinks Roger is being insincere or sarcastic?