Because junk food doesn't contain the protein or complex carbohydrates that your body needs to maintain consistent blood sugar levels, your blood sugar levels will drop suddenly soon after eating. You'll crave sugar and likely end up eating more junk food.
Even in the short term, eating too much junk food can make you feel really uncomfortable. It can lead to mood swings and constipation, and lower your energy levels so that you lack interest in the exercise you need to burn off those extra calories.