I would need to learn how to be comfortable with being uncomfortable.
This is true for many different aspects of life. I have worked with thousands of people who suffer with anxiety and panic. They all have to learn to not be upset or frightened when their emotional state changes. Changes occur in new situations, from stress, from fatigue, from confrontations, from confusions, from disappointment, and many other variations in life. This happens every day to everyone. People have to learn to cope.
Being physically stronger is certainly a help in dealing with these variations. Exercise helps the brain in many other ways also.
But, I disagree with some of the training aspects of learning to deal with pain. Yes, there is some pain and discomfort in training, especially in the early phases. But real pain is a message your body sends you when you are doing something wrong. Marathon running is certainly not good for everyone.
Real suffering is not a character builder; it can be a destroyer. It is good to push yourself. It is good to learn new things, go new places, and meet new people. But people learn more from success and positive outcomes than from torture and deprivation.
Also, I don’t think that a lot of extra physical training translates into better relationships or other better habits. The discipline you learn from running up mountains by yourself is very different that the skills you need to get along with others. I feel that the socail skills such as understanding the motivations of others, along with empathy for their situations are more important to living happily and successfully. Humans are social creatures. Spending too much time and effort on yourself alone becomes counter-productive.