Last week a client confessed that she had to eat "something bad." It made her feel a bit rebellious. As a teenager she was constantly told "what to do." I find that this thinking is not uncommon with overweight adults. In fact before putting the food in their mouths, they may have a "Should I? ---Shouldn’t I?" debate.
It is not that you don't know it's unhealthy. It's more like a compulsion to eat it. When a person labels food as "good" or "bad"; it's easier to feel the need to be a little naughty. Food is not good or bad; it's an item in a grocery market. A better approach (that seems to work) is to think of your food as either healthy or unhealthy. Ask yourself "Is this healthy?" You wouldn't give your child something unhealthy would you? Would you intentionally increase your chances of cancer or heart disease? When you eat unhealthy foods you are doing just that.
Go ahead a bit rebellious in your life, but not with your health.