Processed Foods: If you look at the ingredients of some common place food items, you realize that they are full of all sorts of chemicals. Now most of these are not naturally occurring and the body probably doesn't know how to react to them. What it ends up doing is responding to the alien substances by treating them as toxins and storing them fat (most toxins are stored in adipose fat tissue).
Sugar: High fructose content => massive insulin response => storage of calories as fat.
Wheat: Also a mild toxin for the body. Causes mal-absorption of essential micronutrients. When the body is not getting enough nutrients, it needs to eat more, resulting in a caloric surplus.
Quality of Food: The nutrient content of fruits/vegetables is 50% less then even 30 years ago. Feedlot meat is known to be raised with hormones, anti-biotics and an unnatural diet. What this means is that the nutritional value is not the same and perhaps we have to eat more in order to receive the same quality of nutrition.
Room Temperature: In the modern world most of us are indoors most of the time where the temperature is usually a comfy 23-25 degrees (that's 68F). If the temperature were higher or lower your body have to work harder to cool or heat itself respectively. When we stay indoors and dont expose ourselves to the natural extremes of temperature our bodies expend less energy, resulting in storage and hence fat gain.