It unfortunately can be pretty simple. Even if you think you eat well, it is likely you are eating many calories you are unaware of; or have underestimated. Unless you have counted macros or calories do you really know how much you are eating. This is not to say you should always be tracking, but occasionally tracking lets you know how much you are currently eating and teaches us what real portion sizes are.
Without an objective idea of how much food we need to eat everyday, it can be difficult to honestly assess our food intake. The good news, it is easier than ever to track food intake. Apps like my fitness pal, fatsecret, my plate, make it easier to log food with having very big libraries of popular food choices.
Think about how much you eat or need to eat. Then track your food for one week. Be honest with yourself. At the end of the week look back and see how close you were. It can be eye opening to see where extra calories are. It can be a great first step in eliminating some calorie dense food choices.