Humans often dream about what they like to call a perfect world - no need to work, sleep until noon, anything they want to eat or drink, limitless sex, and no rules regarding indulgences of any kind. They strive toward this, reducing their work time and increasing their play time, and moving the menu in the direction of the fun foods, desert first. A lack of exercise, which at first feels good because one feels rested rather than weary, can result in a very unhealthy body. Likewise, fun foods, a.k.a. junk food, can be just as destructive. Junk food is just too much of a good thing. Cave men lingered longer at the berry patch because the berries were sweet, and thus got their dose of vitamin C. They ate fat laden foods with gusto, and thus stocked up on calories before the lean times set in. And they ate anything salty they could get their hands on, because they were always on the edge of being depleted, especially when inland rather than along the shore.
But cave men were kept in check by their harrowing existence. Overindulgence was the least of their worries. Civilization has in essence allowed man to follow his appetite, unchecked. If sweets taste good, then indulge into diabetes or hypoglycemia. If fat foods taste better than lean, then indulge into obesity and cardiovascular disease. If salt puts zing into food, then apply the shaker until your blood pressure sky rockets. Only when health is shattered do most humans take notice. For those who have more than occasional access to junk foods, or even sweet, rich, or salty staples, common sense should be given at least equal billing. Think of how awful you'll feel if fat, apoplectic, and in the grip of blood sugar swings. Drink too much and you have a quick lesson in the comeuppance that a hangover provides. Junk food has a longer curve, but the comeuppance can plunge your life into the grip of misery which won't be cured in a day.