Pier pressure, man. PIER pressure. Your friends come along and they're all like "Yo, Ding Dong Man, Ding Dong Yo!"
And "Hey man...what is it with you? You on Some Kind of DIET? Is that what they teach you in that little sissy school of yours??
And "You ain't down wid us no more! You ain't fat...you ain't FAT!"
Oh wait...I may be thinking of the Weird Al video.
More realistically, though, friends come along and they, meaning well, want to go out for lunch at, say Long John Silvers. You know, like you've always done. And it SOUNDS nice. And you LIKE hanging out with your pals. And you don't want to change the plans. So you go, right?
The thing about food is, it's a way of life. It's not just what you eat. It's the society you're in. It's how you interact with people.
The best I've managed to muster is to choose carefully what I eat when I'm going out to eat with some pals, and to recognize the temptations that await me when I'm going. I have opted to say no once or twice. And I have chosen the location once or twice. And hopefully, overall, the small changes will make a big difference.