You can bite the inside of your mouth in such a manner that it makes you more likely to bite the same spot again.

Your body is constantly afraid that it's going to starve to death at any moment, to the extent that it stockpiles fat to such a degree that you can die of too much fat stockpile. If you start getting exercise and burn some fat, your body will adjust to try and stockpile fat even harder, so you have to increase your exercise load to keep ahead of the system.

Pain has a emotional component. It's not enough that we are aware of damage being done to our body, but rather sometimes that awareness is so strong that it renders us incapable of defending or repairing ourselves. Meanwhile, any number of other animals can full-on get a major limb ripped right off and they just keep on keeping on like it's no big deal.