most people regain weight after succesfully losing weight - i dont know the details of it but i hear this alot.
maybe as others mentioned sustainability has something to do with it. we eat with our eyes, our nose and our tounges, we see food, we smell food. someone who has gotten fat from eating alot of food will have that desire programmed inside of them, so they can lose weight by trying hard to stay away, but that desire will probably always be inside them, and its probably uncontrollable.
probably like looking at a beautiful man or women, and finding them attractive, theres nothing wrong with that what so ever, its natural. so finding food 'attractive' (in terms of taste) is also natural, so thats a problem, its a recurring problem, a doctor on tv said that people who lose weight will always gain it back.
but sustainability is a key thing here, if you can lose weight in a sustainable way, then your onto something, any other way, then you are sacrificing, and that will require decent amount of motivation, and as we know motivation can fluctuate from good to bad at different times, that is probably why we are on this site right now.
e.g. i dont drink or smoke but i drink alot of coke and have the ocaisonal fast food. because having fast good is a part of my life, part of my social life, and i enjoy the taste of fast food, i probably could give it up for a limited amount of time, but it wont make me happy. but at the same time i know if i just work out a little harder, mantain my weight lifting regime then that fast food will be a sustainable part of my weight loss.
and you should combine your dieting with exercise, your weight loss will be very efficient. you'll be able to burn of so many more calories by making your body work...