A lot of people also aren't "in tune" with their bodies, especially those with weaker autobiographical memories. Many just don't internalize or remember how they feel over different time periods except for extreme events (emotional, strong smells, etc.). Unless they have a hangover, they don't much consider how they feel on a relative scale, let alone try to tie it back to their past diet or lifestyle. Throw in a lot of drugs like caffeine, antidepressants, pain killers, depressants like alcohol, stimulants like ADHD meds and it results in a perfect storm where subtle signals that many people are naturally mindful of just get drowned out. Even becoming aware that you can differentiate signals takes practice just like any other muscle.
This has been my biggest struggle with health. I've always had troubles with autobiographical memory and without it, I can't stay motivated because I quickly forget how good exercise felt or how junk food made me feel a few hours later. I started going to the gym to dogfood when I joined a fitness company and within a few months I was looking a lot better but not feeling it. It wasn't until I started to really push my limit with rowing workouts that I felt the "runner's high" athletes talk about. From that point on, it was felt like I was discovering a new muscle group every few weeks and correlating how far I rowed in a 60 minute rowing session with my diet and lifestyle. Junk food that I would scarf down because unappetizing. Drinking alcohol went from a no-brainer to "how will I feel tomorrow?" Morning stretches and exercise became mandatory just to feel "normal."
Even now, with the gyms closed for months, I can feel that newly acquired skill slipping and predictably, my self discipline slides too. I've almost forgotten how good it feels after a nice workout.