When I curse, maybe 98% of the time the reason is just making something I say funnier (or trying to make anyway).
Maybe 3% of the time when I curse because there's something awful going on, I use the f-word.
I need to be reeeallly pissed to slip that one out!
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EDIT:
I once heard about a research. Two guys had their hand put in ice. The other one swore lots, the other one less often.
They learned that the person who curses less often gets to release more stress through cursing than the other.