I believe the need to challenge ourselves is pretty innate and has everything to do with the way we thought back in the day as tribes. When humans lived in tribes, there was always a threat of other humans coming along and taking your s**t. Maybe it was predatory animals coming and eating your s**t. Being on the ready, the physical and mental ready, is pertinent. There is a part of our genome that has this very beautiful feedback loop. Where if you challenge yourself more, it gets satiated. It feels good to challenge yourself. It is nice, dare I say euphoric, to overcome a challenge.