Enough grudge holding and soon you'll see more iniquity than actually exists. The injustice collector becomes a trigger-happy perceiver. If you walk down the street recounting the affronts you've suffered lately, you'll kick up quite a cloud of dejection. 

Injustice collecting springs from a sensible motive: the monitoring of fairness as a form of self-protection, an impulse that evolved among social creatures who depended on one another. Nursing grudges may have raised our odds of survival and reproduction, however unconsciously. 

Grudge holding😠😠😠😠😠 evolved in part as a means of monitoring the able-bodied freeloader. Our ancestral environment didn't exactly provide job descriptions for each member of a group—we were in a voluntary collective. (Indeed, it's the freeloader who makes enforced collective systems like communism unworkable.)

The freeloader, also known as the free rider, is someone who doesn't pull his weight in the group. Evolutionary psychologists John Tooby and Leda Cosmides argue that we are so attuned to freeloaders, we actually have a specific cheater-detecting ability, designed to spot people who renege on a social contract.