We've all heard that, we've probably even said it a few times, but why is that?
I'm always interested in our habits and emotions around food and I study what individual people do.
At one job I had years ago, a small group of us always worked late on a Wednesday evening to meet a deadline and then went for a pizza afterwards. I would always look forward to this and 'save myself' for it by not eating too much during the day. One Wednesday early evening before we had finished, one of the group a very slim guy nipped out to get himself a burger and chips. I asked him why he was doing that when we would be going out for pizza in a couple of hours. He said "but I'm hungry now". I said "but won't it spoil your appetite later?" and he replied "well I won't probably won't have to eat later".
He didn't even eat all of his burger and chips, stopped when he was no longer hungry and thew the rest away.
I remember realising then the difference between us, I was using food as a celebration and reward and he was using it to ward off hunger and nothing else.
I would summarise then (and very generally) those slim people we are envious of eat to live and the rest of us live to eat.