Interesting. What you are describing is known as the hormetic effect, the idea that we adapt to little doses of stressors such as physical exercise, extreme temperatures, deprivations etc, and this constant exposure to manageable stress makes us gradually stronger, more resilient. (I actually just read about it recently.)

In my case, it's physical training, mental training, that is, the ability to sit still, without moving, for an extended period of time. Also, I used to do fasting.

There's a lot to learn about oneself when testing one's limits, and continuously expanding them. It's empowering, and liberating.