Humour or...
As a History student and an avid loved or the history of Wars, I read a lot of books that depict great battles, sacrifices, victories and great horrors. And once you start reading about the hardships the soldiers suffer on the front, of the decisions made by the generals and of the effects those decision carry with them, of war crimes and the accounts of civilians whose lives are disrupted, distorted and deformed...
I believe ones sense of humour begins to take a hike up a sloppy and dangerous mountain...
I think a similar phenomenon can be observed in Medical students and Nurses in training. Their proximity to death, human anatomy and bodily ailments desensitizes them and slowly twists their humour into something often morbid or even disturbing for those outside of their field of specialisation.
Having lived for half a year with student Nurses and having several Medical students as friends, I say this having experienced first hand discussions that veer outside the norm for conversations, often conversations at the table.
Sadly I do not have friends that are interested in the same area of History but having observed the teachers at both my University and school, I do not think my humour is alone in its loss of sanity.
Maybe it follows an obvious path of degeneration: one reads a book filled with horrible events, considered a norm only in times of war, having been submerged in man's worst escapades, ones minds starts trying to compensate and hysteria sets in. And as result, one begins to find morbid and often completely humourless situations to be filled with glee.