> what is the key to losing weight?

Easy win: No sugary drinks. Juice is as unhealthy as coke. If you want something with flavor, drink tea (without sugar).

For most people, it's to understand this: Carbohydrates make you gain weight. Not the fat in food.

Somewhat hard to implement, but with significant results: Cut sugar entirely from your diet. This automatically results in much healthier eating, because you have to check ingredients on everything this way.

Eat fewer calories than you expend. Most of your expenditure is BMR + walking around, exercise doesn't add that much (unless you're at Olympic-swimmer levels).

I think the point OP is trying to make is that it is very easy to eat in excess (calorie-wise) to what you are burning by training since physical activity will often make you crave more food. Doing some kind of training is obviously good for you, but you will not necessarily lose weight if you don't restrict your caloric intake.

Just like intelligence differs, your ability to gain or lose weight is not the same as other peoples'. In my experience it is not fair, but on the other hand, what is?

I would recommend something like the slow carb diet (Tim Ferriss), but anything that restrict your intake of calories and that you feel you can sustain over a long period of time is good.

Eating less calories than you take in: measure your food accurately and keep a food log, then stay under the computed target for the day.

It works every time, it is just hard to keep up with if you aren't motivated. I know it works though, it help me shred 25kg.