I read somewhere that a study was done, and people who eat instinctively ( without any sort of strategy to control calories ) on average, over ate by 100 calories a day.

3500 calories is the rule of thumb used by fitness enthusiasts to represent one pound of fat. Ignoring the fact that it takes about 2 calories per day to maintain 1 pound of body fat, eating an extra 100 calories per day ( half a can of soda +/- ) would result in 1 lb of weight gain in 35 days, or just over 1 month, without the person ever being aware they are overeating. Carry that forward for 10 years, and theres the extra 100 lbs many people are carrying.

This is a simple example, as other factors like processed vs unprocessed also come into play, but I think the point is clear:

Eating food without accounting for calories, is like running a business without accounting for the money. Both tend to not end well.