To my way of thinking...when we diet restrictively, with no allowance for "goodies", we are punishing ourselves for getting fat. I can't think of one person in my life that would put up with someone else punishing us for a week straight, let alone for months or even years...why would we do it to ourselves? We shouldn't.
That serving size bag of M&Ms is ok. Three of them is not.
When we stop thinking of it as losing weight, and start thinking of it as giving our bodies what they need and being healthy, the weight loss comes along for the ride... :)