This is a simple answer. Fast food is just stacked full of calories. You might think that having a burger and fries looks quite modest in comparison with roast beef and vegetables but there's a big difference in the calorie count. If you eat the occasional - and i mean occasional, e.g. once a month - your weight won't be affected, but eat one most days, and it certainly will.
Best protection is to look in the windows of burger joints and look hard at every obese person who passes you and ask yourself if you want to look like that, as well as dying earlier than you need.
Fast food makes you fat because of the high content of fat in their foods. There is also sugar in them too. Sugar turns into fat. Since there are so many fast food restaurants in a town, it is easy to be tempted to quickly swing by there and order their food. Also, people who eat in their car or who are in a hurry would have an easier time ordering a cheeseburger and French fry opposed to trying to eat a salad in a hurry.
Most of these foods like the fried chicken sandwich, chicken nuggets and double cheeseburger contain as many calories as what a person is supposed to have all day. Fried foods contain cholesterol which also leads to fat problems.