That's a lot of psychoanalyzing of people you don't know.

If it helps give you some perspective, I've been through a decade of therapy and years of group therapy, and I haven't found that my interest in "discipline" lessened or became less relevant as I've come to grips with emotional issues and learned to relate to people in different ways. There hasn't been much interaction between the two, except that when I'm doing well at one I tend to do better at the other, for the obvious reasons.

An alternative explanation is that voluntary suffering is a comforting reminder that you can do it when you need to, as well as an effort to maintain that ability. My observation is that people who engage in "gratuitous" suffering are people who were raised by parents who didn't pass on the basic skills of discipline (among which are important emotional skills) and had to learn it from scratch later in life. Since they didn't grow up with it, they don't take it for granted, and they exhibit the "zeal of the convert." Or, if they never achieve it, the persistent lack keeps it at the front of their minds.

The parent poster seems to have quite a lot of discipline but doesn't feel any need to remind himself that he has it. Maybe that's because he learned it as a child from his father?

My father had a hard and restricted life growing up, and he wanted me to grow up in a more free and easy way. He wanted me to feel free to goof off and enjoy myself. I certainly learned that, but I also grew up with a lot of anxiety about my inconsistent ability to apply myself to things I cared about. Sometimes I did, and thrived, but very often I disappointed myself. I was especially poor at boring things and things where consistency mattered, such as taking care of my health, but often, even at the things I was good at, I failed to put in effort at a crucial time and screwed up something that was important to me. My easygoing father wasn't going to tell me I "lacked discipline," but my frequent underperformance and self-failure told me loud and clear.

So I spent years figuring it out and getting better at it. Like anything that doesn't come "naturally" (i.e., wasn't learned in childhood) it has always felt like an artificial bolted-on part of myself that could fall off at any moment. So I consciously tend to it. I don't show more discipline in my life than other people, but I certainly show more interest in it, and I do some things that probably seem weird to other people. Other people probably think it's a meaningless focus that has no impact on my life, since I'm not conspicuously different in my habits and accomplishments. But not being conspicuously different is the accomplishment!

A lot of people can relate to the effort required to maintain a healthy weight after growing up overweight. People who take a healthy weight for granted might think that an above-average amount of attention and effort should yield an above-average physique, or what's the point? They might feel amused or condescending towards their friend who thinks and talks so much about what they eat but looks very average. They might get annoyed that their friend declines an extra round of drinks or skips dessert. What's the point? It's not like they're maintaining a fabulous physique. They look like anyone else, even slightly below average. Clearly their obsession with eating doesn't accomplish anything other than annoying the people around them. But it does accomplish something. It lets them be closer to normal.