I understand this. It's just that my brain is a bunghole, and has decided that any maintenance task that the body may need is about the most boring thing it can imagine, and it would rather die than be bored. The body retaliates by pumping the brain full of chemicals that make it want to eat.
The perfectly rational part of my brain has set up activity reminders at work for 11AM, 1PM, and 3PM, presuming that walking to/from the parking lot counts as activity. They literally just say "Get up and move." The lazy jerk part of my brain always responds, "...or don't, and just click off the reminder."
And sometimes it wins. It shouldn't ever win, but it does.
Ultimately, the problem must be that part of me doesn't want to live a long and healthy life. It would prefer to die sooner rather than later. Tracing it, it seems linked to the future prediction part of my brain, which seems to believe that the future will be worse than the present.
I can't really fault its reasoning. If I knew civilization was going to collapse next week, I'd quit my job and spend some time partying. If I knew it was going down in a month, I'd probably check out at work and party at a more sedate pace. So I think I can understand one of the reasons for the US opiate epidemic. For those people, the world as they know it is ending, so they're checking out and partying. Their job is probably gone and not coming back, and maybe no one really cares about them anymore, if anyone ever did. So they can face a long, slow, downward spiral of pain and despair, or they can jump off a cliff and make it a quick rush of euphoria, then no more pain.
Sedentary lifestyle is more of a response to a general malaise. I sometimes don't think I'll ever be able to retire. Most of the jobs I have held have absolutely sucked. And many of my ancestors were at least octagenarians. Rather than spend the next 40 years working for The Man, part of me might be whispering, "being dead is a great excuse to stop working so hard for the exclusive benefit of other people."
So essentially, I probably don't change my habits because I believe that living longer would just allow other people to use me as their pack mule for more years. I don't really want to die, but I haven't really been living for the last 20 years, either. I earn like a software pro, spend like a stingy hobo, and have nothing to show for it, not even social capital.
Why would I invest in myself without even the possibility of getting equity in return? And that's just a really difficult argument to counter. What could I do to get more enjoyment out of life, that would make me want more of it? If America has epidemics of drug addictions and obesity, what does that say about its opinion of the future? Are we all a Rat Park Experiment writ large, pushing our levers in our Skinnerboxes just because there is nothing better to do?
The world needs something new to believe in and spend money on, but I'm not charismatic enough to give it my dreams.
I may be overthinking this.