There's little that causes genuine laughter like genuine laughter.
You've seen it. When SNL actors start to laugh in the middle of a scene. It's a mistake but it's supremely enjoyable. We imagine the fun it must be to do what they are doing and we watch them enjoying entertaining us. It's always funny.
I'm currently directing The 39 Steps, a very funny parody of the spy-thriller. And it's never funnier than when I notice the actors struggling to keep from laughing. I don't encourage it. It can hurt the narrative to some degree. But when one actor does something hilarious and it takes the other off guard, it's hard not to enjoy the stifled giggles and covered smiles.
I've experienced this quite a bit myself. I have done a lot of improv shows and specifically, I have often hosted those shows. And as a host I find that the audience is often watching me watch the scenes. I don't think it's because I'm ridiculously good looking, I think it's because I'm their proxy on stage. I am the first audience member and I am often cueing them about how to react to scenes. When I laugh, they laugh, both at the players, and at me laughing at the players. After shows, audience members often want to talk to me about moments they saw me laughing the hardest. I've even heard them say "I love watching your reactions to the scenes as much as the scenes themselves."
I think there's an honesty to laughter. Fake laughter is immediately recognizable and, I daresay, truly off-putting. But someone caught up in a moment of hilarity is true, honest, vulnerable, and very human. And, as I always tell my actors, there's nothing more compelling onstage than someone being fully human. Laughter breaks through the barriers of self-consciousness, especially spontaneous laughter like in the clip above.
Perhaps it's why we spend so much of our time trying to make each other laugh. We are trying to spur an involuntary joy response. Because we know it's real.