I think most people gain back the weight because they haven't learned to eat properly. They just put themselves on a restricted diet until they reached their goal. Once they hit that magic number, they went right back to their old habits. The problem here is education from childhood about proper healthy foods, not about "dieting doesn't work." Dieting does work for what it does, but after the diet, you need a skinny lifestyle if you want to keep a skinny body!
It's hard to make the first change, but it's not hard once it's a habit to not buy food that is pre-processed. Buy staples and ingredients and put them together.
I haven't even bought salad dressings for a year because I just pour some olive oil, balsamic vinegar, a sprinkle of sugar, a sprinkle of salt and pepper, and some dried basil into a jar and shake it up. Add dijon mustard and honey instead of the balsamic for a change.
Seriously, it's way cheaper, healthier, and super easy. And fewer jars to clutter the fridge door!