There's this idea in biology called "homeostasis". Basically, your body has an idea of chemical balance, and it adjusts to stay balanced. Like: it's hot out, so you sweat, so you cool down, then you stop sweating. Or you eat candy, which raises your blood sugar. Your body will release insulin, which will lower your blood sugar. As it gets back to normal, insulin release tapers off.
This applies to everything. If you drink alcohol once, it'll make you drunk. If you drink it every day, your body will start adjusting to compensate for it, and you'll have a harder and harder time getting drunk. Soon alcohol just brings you up to normal. If you then stop drinking, your body will now be off-balance in the other direction, and you'll go into withdrawal until homeostasis can be restored.
Pain? Similar deal. Your brain tries to maintain homeostasis. Acute, agonized misery is only sustainable for so long. If the pain itself can't be dulled down, your emotional reaction to it can be. The emergency shutoff switch is when this doesn't work, and all your emotions are cranked down as far as they can go. That includes distress, pleasure, and fear: basically, it causes apathetic depression. This is why you can't just expect people to "get used" to severe pain.
When I say this applies to everything, I mean it. Sex, gambling, sugar, background noise, everything is filtered through your perceptions in this way. Why aren't rich people all happy? Because that level of prosperity is their new normal. What's up with Stockholm syndrome? Victims become desensitized to abuse, and their expectations for "kindness" are steadily reduced. Why aren't teenagers overwhelmed by the pace of the internet? Because it's normal for them. Why don't better football helmets reduce the number of concussions? Because people just do dumber things in order to maintain the same sense of acceptable risk.
This one principle explains quite a bit about why people act the way they do.