Why do people laugh in serious situations? I've seen people (including myself) that laugh for no apparent reason in really serious situations, such as in an argument or when receiving bad news. Although the other party is clearly very upset, it seems they have the worst possible reaction: they start laughing. It's probably not because they find it funny, so what does trigger it?
This could be what the Psychology Today article "Why We Laugh", (Lickerman, 2011) refers to as 'nervous laughter', suggesting that this response is both for reassurance (as suggested by Tyler Langan's comment) and also a means to build resilience in the face of potential trauma, specifically (from the article):


  This may explain why some psychologists classify humor as one of the "mature" defense mechanisms we invoke to guard ourselves against overwhelming anxiety (as compared to the "psychotic," "immature," and "neurotic" defense mechanisms). Being able to laugh at traumatic events in our own lives doesn't cause us to ignore them, but instead seems to prepare us to endure them.


Further, it is suggested here, that the nervous laughter is a means to protect our dignity and sense of control.