Life is an amazing journey. There are all types of experiences one will face. To some theses experiences good and bad , will be causes for degradation. To others they will be causes for growth.
Why would anyone allow oneself to degrade is an amazing proposition but knowingly or most likely unknowingly they do so. It is natural like gravity pulling things down. It seems natural for people to think negatively with a win lose mindset. It takes great effort to think positively and with a win win mindset.
The reward of the latter approach to life is very subtle and long term. So people don’t choose it even if they know it.
If they did they’d be in perpetual bliss like me even in the worst situations.