Why do people say R.I.P?

Yeah, those Middle Age traditions... like engaging in self-flagellation as a group in public, to atone for the sins of that town, all the while spreading the Black Plague with the resultant blood spatter.

I always say, when someone sneezes, "Gesundheit", translation - 'good health', except around one friend who has been obsessed with the horror of the Holocaust her entire life.  She's still pissed at those damn Germans, and even though I am 1/4 German and speak the language, I can't around her.  It's a real serious deal with her.  Her father fought in WWII, in the Battle of the Bulge, and I guess he somehow communicated that horror to her, though he never spoke openly about his time as a soldier.

Humans are just hard-wired for tribalism and conflict.  And belief in living forces that we cannot see.