I once read somewhere that ocd is like a broken gearbox in a car, and the different gears are different thoughts. Whereas "normal" people can easily shift between thoughts, people with ocd are stuck in gear 1. They are stuck on a thought that is really not that important, even more, that is completely irrational, but they don't seem to be able to shift to the next gear (another thought). Maybe because the oil (serotonin) is missing? So in order to continue their itinerary, they try to use force(= resistance) to shift to the next gear, which isn't working obviously (it damages the gearbox even more). What they should do is relax and gently move the lever (= acceptance) to the second gear, ignoring the terrible noises coming from the gearbox.