I'll answer from a US perspective, since that's where most of the studies I've seen come from, coupled with working in the US system.
I'll point out a few areas the statistics possibly show issues. There are less than 15,000 controllers in the US(estimated, from memory and older sources). Most other professions usually are far larger, and such that an occurrence/issue in the controller population often pushes it higher in statistics.
But, it is a very demanding job, and a very demanding training environment at a lot of facilities. In the past, if you had 3 losses of separation, within a year, you would be penalized big time, to either a lower facility, or to firing, this increases the stress level in the past studies. The punitive nature has changed to a different system, that doesn't fire someone, unless if it was blantant disregard of the rules, but an incident(loss of separation to accident or any thing else) still can take the toll on a person. In training, you're pushed to be perfect, in terms of spacing, to phraseology, to procedures, to complex airspace, to vast rule books.
The rule books, if printed out, are generally 500-700 pages each. There's ~3-4 main ones, and tons of little ones. The rules are in a continous change state, and where one thing is legal one day, could be completely changed over the course of a week(rare, but it has happened). Plus each facility has numerous procedure documents and letters of agreements with facilities or agencies that they have to work with. This makes a controler have to have a vast knowledge of rules and how they all interact.
Routine situations, even with lots of traffic become easy to handle, once you know what to do and when. It's when the **** hits the fan, that it becomes stressful, whether it's student pilots, language barriers, emergiencies(and there's various kinds that either don't phase you or those that can completely wreck you), to the worst thing, weather.
There's other reasons that it can be highly stressful, from medical rules, to just raw work rules and management issues.