why they gain weight

I have always been an active person doing many things outside when not at work. I have wondered for several years why people gain weight and begin to get fat. I looked in the mirror about a year ago and it occurred to me that I was fat. It seems it crept up on me when I was not watching. Working the kind of job I work now I can see why people (even me ) get fat.I work 12 plus hours per shift and when I come home at 8pm I am to tired to exercise, but I eat dinner and then I am ready to go to bed. In doing this I now know how easy it is for one to get fat. Maybe the reason I got fat was so I can have more compassion for those of us that it snuck up on when they were not watching. Thanks for listening.Steve