From working with clients and from my own personal experience, I've found there are two primary reasons why eating a cupcake makes someone feel bad about themselves.
Some of us live in a constant state of "dieting." We count and restrict calories, or shut out entire food groups (no carbs!) in a weight management battle that rules our lives. These individuals label foods as either "GOOD" or "BAD" and if they eat from the "GOOD" group, they have permission to feel good about themselves. But when they eat from the "BAD" group, watch out. They're going into a guilt and shame spiral. They feel like they are weak and out of control. Their worlds revolve around this system of "GOOD" and "BAD" eating, and eating a cupcake doesn't represent celebration, it represents failure.
Others of us have diets that are full of salty, sugary, fatty, heavily processed foods that provide plenty of calories but not much in the way of real nourishment (and yes, there is a BIG distinction between calories and nourishment!). Though a cupcake is supposed to be a treat, a symbol of celebration, for these folks, it's just another blip on a landscape of poor nutritional choices. Not much to feel good about here.
Food is fuel, that's for sure. But it's also pleasure. It's a way that brings humans together. We eat foods to celebrate. When we deny these latter aspects of food, we miss out on some of the best parts about being human.
If you are ready to make some changes to your diet and lifestyle, and to actually enjoy a flippin' cupcake every once in a darn while, let's talk.