Human nature. We are programmed for two things to survive. One, eat food when it is available. And two, conserve energy as expending it requires more food.
It hasn’t been that long since we had to forage, fight, hunt, completely prep, defend and repeat for food everyday. The brain has two very strong reward mechanisms. food and love. When we get either, our brain goes “Woo Hoo!”. And with food, the brain also says “more, more, more” for most people. Being able to turn that off is something that can be learned. But it takes a strong focus.
Then there is the conserve energy side. People generally prefer to lounge whenever possible. In times of abundance, people have historically partied. And if food was tough to come by, as soon as a belly was full, laying down to sleep was the next thought. Even mechanically, the brain attempts to get things done with as little energy as possible. There is something to “a body at rest tends to stay at rest” for people as well. But just like not eating everything in sight, we can learn to enjoy activity. But again it takes time to acquire that mindset.
And falling off the horse usually means starting almost from the beginning on either issue. But as both are learn-able habits, repeat attempts usually become easier. As long as there aren’t any major disruptions physically or emotionally.
For our part, we just need to be the stead support and source of information that we dreamed we would be when we became fitness instructors. (And that works for us as well. We could all use a little support, a little faith, and a little success to keep us on the path.)