I think a lot of it comes down to what made you decide to change. 10-12 years ago I decided to get in shape, and I calorie counted and exercised every day for the next 2 years, and stuck to the gym for a decade. Now I'm struggling to get back into it again after letting my weight creep up.
What made me start the first time was a really scary moment of getting up and nearly blacking out (not getting up to fast and getting dizzy; I'm talking things just went properly black in a way I've not experienced before or since) and having distorted vision for 20 seconds or so after I sat back down.
Couple that with persistent knee pain that made my doctor look at me for about 10 seconds before giving me an exercise sheet for old people (when you're in your 30's and get handed a sheet of exercises where the pictures are of people 60+ it's not a good sign).
Get the right motivation and it gets a lot easier to stick with it.
Different life events made me start to slip a couple of years back, and at the moment I find it hard to be motivated in part because I still feel healthier than I did last time, and I'm far stronger than I was, and feeling like I'm in a better position makes it so easy to justify that one extra helping of food.
I'm not saying that to diminish the effort it takes to stick with it, but just to point out that even for the same person, the right catalyst (or lack of one) can mean the difference between sticking through it all the way through and not even getting started.
Also keep in mind survivor bias - not many of us will write about how we hoped to drop a hundred lbs but failed 5 lbs in. 10 times in a row.