Bread is good, although it is not terribly exciting in and of itself. But it fills us and keeps us filled, and although it doesn’t give us the sugar rush that accompanies a doughnut or candy, its energy stays with us. In short, bread is good, solid, if somewhat ordinary, food that gives us what we need to live and work.
Junk food, in contrast, is about flavor, not function. The idea is to titillate the taste buds now, with little concern for sustaining us. Indeed, it usually has a negative effect, both through the crash that quickly follows its consumption and the ways it degrades our health over time. As good as it may taste, you cannot live (for long) on junk food.