Keeping it off.

Yep, because people think "on a diet" not "I'm going to change the way I eat, which was the cause of getting fat in the first place, forever".

People who are not overweight don't scarfe down 3000 cals a day + and stay trim. They eat less and do more.

People who do scarfe down 3000 cals a day + and do very little - and think they can cut calories, lose some weight, and go back to that lifestyle when they're done are of course going to get fat again. That's why they got fat in the first place.

I had to change my lifestyle instead of "being on a diet". I once lost a lot of weight just to end up where I started. Now I have changed my lifestyle. As long we miss the "old days" of cookies, beer, cakes and being a couch potato we are doomed to fail in the long run. I can speak out of my own experience.
My diet is my new lifestyle. Instead of going to the pub twice or more often a week as I used to I only go out on special occasions what is usually around Christmas or when I am invited to a due.
Now I like my gym. It's a place I exercise AND socialize. I eat differently and do enjoy it.
It's a change for life.
Other things what have changed as well: Even I spend more money on my grocery bill I spend far less in bars and pubs and have more money. I made "better" friends. I am more active and enjoy it. I started a new job and have a better pay grate. I am less depressed. I enjoy the little things in life more. I am more happy with myself.