Medical conditions, like hormonal imbalances: This can be determined in most cases with a blood test from the medical doctor. This is usually a hereditary condition, therefore the doctor will usually be happy to check for it.
Medical conditions caused by the client being overweight: Some conditions can be triggered by excess weight in some cases, such as diabetes. With this condition there are a number of possible causes, one of which is long-term excess weight. Indeed, recent studies show a clear link between childhood obesity and diabetes in later life (in no way are we saying this applies to all sufferers!).
Greed: Some people simply overeat. As to why, this can be seen as a personality trait, or an emotional reaction.
Comfort eating: The person suffers from a range of emotional issues, often related to self-image, and responds according to one of a number of patterns, resulting in overeating, eating inappropriate foods, or indulgence in addictive foods and drink, such as alcohol.
Ignorance: In some cases the person simply does not realise how to eat sensibly. This may be because of a lack of education, caused by an unusual upbringing, even possibly because the person has a learning disability.