What about Hypothyroidism? That slows the metabolism, it can always cause insulin resistance and other problems that keep a person retaining weight. Hypothyroidism IS NOT a result of being fat. -- Medical problem. Very Common. Hormone problem.
The fat gene, I already posted a link to it as well as informed you how to google. --- Medical problem. Rare, but is real.
Emotional and psychological issues can cause one to gain weight, because everyone handles everything differently. -- semi Medical problem, more so phsychological. More common than you'd believe.
Polycystic Ovary Syndrom, since you arn't a woman, I don't expect you to wrap your head around this. But it can cause insulin resistance, which in turn can cause a female to retain weight. It is a particular mutation of a gene when you are born in some cases. It is NOT formed because you are over weight, it makes it harder to lose weight. -- Medical Problem. Very common. Hormone problem.
Injuries, such as of a major part of the body. Back, spine, leg, etc. It can leave one with the inability to get out and exercise and some people, do pack on the pounds easier. -- Not so medical, but accidental and happens. Actually, any surgery in general.
Over eating, due to lack of information about nutrition. -- Common problem, but is usually saught out.
As far as skinny gluttons. They are very real. I have friends who weigh 90lbs soaking wet and they can eat and eat and eat. I have a buddy who is very skinny and lives off a diet of soda and fast food, all day long.
Did you know there are very healthy over weight people? Yup. Good blood pressure, good systems, good LDL and HDL levels, no insulin resistance.