Why is it so difficult to lose weight?

I am asked this question at least a dozen times a week. This is invariably followed by - I must have a hormonal problem, doctor, can you not find it and fix it?
My stock answer is always - "If I knew why it is so difficult to lose weight, and if I could fix it, I would be richer than Bill Gates". I was asked to give a talk on this topic sometime ago, so these are a few very interesting points I got while researching this question. Please note, I AM NOT OFFERING QUICK FIXES FOR WEIGHT LOSS. If that is what you are looking for, read no further. 
There are are some reasons why the body is reluctant to let go of the fat (or adipose tissue) that it has gained. On top of this, there are reasons why some people lose weight easily, and others struggle. Some reasons are:
1. Men lose weight more easily than women. There is a wonderful, small study done that followed men and women who exercised similar amounts. 90% of the men lost weight. Among the women, a third lost weight, a third remained the same, and a third actually GAINED WEIGHT! Granted, this is only one small study, but it bears out what we have seen in real life. Wives get very frustrated when their husbands lose weight easily, while they struggle. This may have to do with the muscle mass, and testosterone levels. This is why, it is good for women to build up muscle mass, which helps them burn more calories. 
2. Our metabolic rate tends to go down as we get older. At the age of 50, we have to work at double the intensity of when we were 35 to lose the same weight, or even to maintain the weight and prevent it from going up. 
3. Brown fat is a wonderful fat that each one of us has in varying amounts. It is called "good fat", as it helps increase the metabolic rate and burn more calories up. All of us are born with varying amounts of brown fat. In some people, more of the brown fat remains, and this helps them remain thin, no matter what. When people say "she has a good metabolism", it really is true if she has a lot of good brown fat. These people either do not gain weight, or lose weight faster with diet and exercise. 
4. There are some genetic differences in people that make them respond in differing ways to even weight loss medicines. Certain variations of genes have been noted to help lose weight and respond to medicines better than other types of gene. If your family is tending towards the overweight, so will you.
5. Studies have shown that we tend to underestimate what we eat, and overestimate our exercise. Net result - we are eating more than we should, and exercising less.
6. Consistency is key. If we lose weight at the rate of 1/2 kg per month, we are doing well. Slow and steady does win the race in this situation.
Finally, most important, weight does not matter. I have decided to stop measuring peoples' weights in the clinic, unless they tell me they feel they have lost or gained a lot of weight. What is most important is - are you following a healthy life style. Are you eating well? Are you exercising your mind, body and soul? THE WEIGHT ALONE DOES NOT MATTER. Throw away those accursed scales!
The reason I say this, is people get hung up on the number on the scales, and forget about what is important - which is living a healthy lifestyle.