system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people develop tolerances to things like pain and extreme cold? What's happening at the biological level for it?
gpt
The body, and more specifically the brain, is highly adaptable and quickly tries to ignore what it sees as irrelevant information. When you first sit down in a chair, you may feel aware of your butt on the seat but after a few minutes of sitting there, you stop being constantly aware of that sensation. Another example is drug tolerance. If you use heroin all the time, your body and brain adjust themselves and your neuro receptors to this new state of 'normal'. When you take away the heroin, your reconfigured physiology is not adjusted anymore and you get withdrawal symptoms. 

Acclimatization takes advantage of this. Repeated exposure to something noxious or harmful causes your body to change and adapt. Consistently causing yourself pain builds up a pain tolerance. Another good example is the USA Olympic Training Center which is located in Colorado Springs. By training in an environment where there is less oxygen, athletes' bodies become more efficient at using oxygen so that when they come down to lower altitudes to compete, they perform better. 

Obviously there are limits to this phenomenon and there is a wide variance of the ability to acclimate among individuals. Some people are more tolerant of pain or cold to begin with and like everything else, genetics plays a role.