Do you know any one who quit smoking and started eating more and gained weight?. In the contrast – what about some one who started exercise and lost the desire for eating lot of food or became a conscious eater?. Or you may pick up a new social activity when you stopped some other social activity.
Going back to our foundation premise – we do things to feel good or to avoid pain. From the first example – when you smoke, your brain produces a chemical called serotonin (which is a feel good chemical). So when you quit smoking your brain will push you do some alternative activity which can produce the missing serotonin. So the easiest thing is to eat food. And people around and including yourself will think this is better than smoking so you would treat eating food as a reward.
Same pattern is applicable for all habit switching. But here is the trick. When you are switching from a “bad” habit you need to replace with a “good” habit. Both good habit and bad habit will produce feel good chemicals. For e.g when you switch from “smoking” you need to replace it with “regular exercise”. If you are lucky to start with a good habit like exercise then your serotonin kicks in so you would not need alternate thing like food.
In a way we all are addicted ( to feel good). Having right addictions will help you and put you in the forward motion.