Just got a text about this... "did you ever notice that if you mention to a co-worker that you are on a diet, only to have them pressure you to try the cupcakes in the staff lounge?"
I have noticed over the years that if you are ever going to try to get any competition going in an office or home, start an exercise competition. With exercise, people will strive to exercise more than each other. While if you start a diet competition, generally people will either "cheat together" or will try to knock each other off their diets, really! And if you didn't think the mere mention that you were dieting meant you were starting a competition... WRONG! LOL
Is it human nature?
I think it is that exercise is a positive thing in that it is something that we can compete at by DOING more than someone else.
While dieting is a negative thing in that it implies that you compete by not doing something, like eating fewer cupcakes than someone else. "Eat Fewer" just doesn't make sense to an action oriented person the way "walk 20 minutes" does.
Anyone can see that the way to win at this sort of game is to make the other co-worker eat more cupcakes! :)