Filthy hands, long nails & fingers that dig like they're looking for treasure in between the couch cushions are my top three reasons. I have no cervix, but that doesn't stop some men from thrusting their pointers towards it! There's also an acupressure point that many men hit by accident when they are doing digits. If they hit it lightly, my right sciatic nerve tingles. If they hit it hard, it's painful from the crest of my hip down the back of my leg! When a man with short nails & clean hands that knows what he is doing hits all the right spots, it's terrific. The rest of the time? YMMV...but if I start saying OWWWW instead of AHHHH, better reevaluate the technique, fella!