So let's just start by stating the obvious: your body needs food.
Slightly less obvious: Your brain needs food, too. Just because your brain isn't a moving part doesn't mean it doesn't need energy. In fact, your brain uses about 20% of your total caloric intake.
So when we are stressed, why might we eat more? The research is unclear about whether or not high brain activity requires significantly more energy, but either way, having low blood sugar levels isn't going to help you think clearly when you need it most. So in response to a need for greater focus, our body may send us into a consumption mode to make sure we are fueled up.
Our bodies are meant to chew, but with the sort of food we have widely available now, we can get to our daily caloric needs in a lot fewer bites than what nature intended! What kind of food would be available to a stressed out person living 200 years ago? Okay, NOT refrigerated cookie dough, Big Macs, Doritos, and microwave popcorn. So if our bodies are naturally wired to want to munch more during stress, the kind of food we are responding with is like putting out a candle with a firehose.
But let's be real, if you're stressed, the last thing you are likely to be when it comes to food choices is TOTALLY LOGICAL. I'm not. And this goes double when I haven't had enough sleep--which is also usually a factor when we are under stress.
At times of stress, there is usually something that we are supposed to be doing that we don't really want to do. Maybe we have to deal with some paperwork, or an unpleasant relationship, or tackle a difficult chore or project. Instead of doing what we SHOULD be doing, we reason that we need to fuel up first. Before you know it we've mindlessly eaten a half-batch of cookies because that requires WAY LESS brain power than preparing our taxes. And you know, the stress doesn't even have to be that big. Maybe we just feel tired, or don't want to do the dishes, or have to deal with putting the kids to bed AGAIN.
So it's not necessarily wrong if we feel the impulse to munch when we are feeling stressed. Go ahead and feed your body, but feed it something smart (with a mix of carbs, fats, and protein), and then *move on* to doing that thing that you are avoiding. Maybe then you won't be so stressed anymore! Problem solved!