Main reason is that they don't know this, so they relapse by accident. This fits very well of people gaining weight around the holidays, when you're exposed to

Main reason is that they don't know this, so they relapse by accident. This fits very well of people gaining weight around the holidays, when you're exposed to lots of delicious food that's outside the daily diet. And once they do this, next time they're hungry you can bet that vegetable rice will not be the first thing that pops into their heads. Doesn't help that we tend to starve ourselves before big meals out of guilt, which only makes the habituation stronger. Another reason is the "voluntary" part. It helps a lot if giving up food is a choice, as opposed to something imposed by a diet. If you see cake, make a cost-benefit calculation and decide that it's nice but doesn't fit into your lifestyle, you're likely better of than if you decide one day that cake is forbidden because it's bad.