"Unhealthy" foods are generally very high in fat, sugar, and starches. In other words, they are full of energy. That's not necessarily great when you eat a lot of that food and don't exercise much. But in the past, energy-rich food was scarce and very valuable to survival. So our bodies crave it and get a big rush of happy brain chemicals when we eat it. Evolutionarily, "unhealthy" food was actually really healthy food if you could get your hands on it - again, because it was usually in small amounts and really gave you a survival benefit.