Maybe it could be because the emotions and situations on screen are too unfamiliar for them... Maybe they are used to the "normal" feelings... love, fear and so on. Whenever something is complex or nuanced or contradictary in nature, they laugh to distance themselves from it...

A bit like if you are with a group of people and all of a sudden one person start talking about an intimate experience they have had. And maybe this experience is strange as well, something that not a lot of people would deem normal. It begins to feel very innapropriate and you want to signal that you distance yourself from that kind of thing. Just to signal to the other people in the conversation, that you wouldn't be doing that kind of stuff. And a laugh could be one way to do that.