There are many people who have come to conclusion that weight-loss is not possible. It is extremely hard to convince them otherwise, seeing that this tends to be a conclusion they reach after their own experiences. Most of the people holding this point of view will tend to be precisely the people who really need to lose weight: people whose body weights put them at great risk of various illnesses, while also considerably reducing their aesthetic appeal. Most of them will be people who have tried pretty much everything that is said to have any weight loss efficacy, without success. But looking at their life stories, it becomes quite easy to identify a number of common threads, pointing to possible causes of failure in the weight loss endeavor.
One of these three causes of weight-loss failure is that of using the wrong weight loss approach. Many of us, upon realizing that we have a weight problem, panic and start doing everything we can to shed off the extra kilos. Unfortunately what we are doing will tend to be far from the ideal approach to weight loss, especially in the particular weight problems in question (since every problem is different, requiring a different type of approach). So a person whose weight problem is one that would be best addressed through dietary modification ends up taking up an improper approach (say use of supplements) and hoping to lose any weight. What you can do about this problem is obvious: you need to work out, ideally with some professional input, what the particular sort of weight problem you face is like, and what the best solution(s) for it would be.
The second major cause of weight-loss failure is inconsistency. Weight-loss, as most of us who have gone through it successfully will tell you, is the sort of a goal where regular and rigorous work is required to attain the desired results. The weight problem is also the sort that cannot be resolved through inconsistent and uncoordinated effort. Whether it is a diet you have to go on, or whether it is a gym attendance you have to opt-in for, you have to ensure that you are very consistent, for you to start seeing any positive reflections on your weighing scales. Most of the people who encounter weight-loss failure turn out to be people who have their inconsistencies to blame for the predicament they find themselves in.
The third major cause of weight-loss failure (as indeed failure in most other important goals) is that which revolves around lack of proper motivation. Unless you can find proper motivation, the difficult steps required to bring weight loss about would feel like too much work. Yet it is not too hard to find proper motivation, as long as you can keep your eye on the prize (what you stand to gain, in terms of health and beauty) upon the attainment of the goals you will have set for yourself, in terms of weight loss.