Why do people go on a diet?

M: Everyone seems to be on a diet, Have you noticed that, Mary?
W: Everyone seems to be talking about diet.
M: A neighbor of mine is on a banana diet. She eats a banana for breakfast, a banana for lunch, and two bananas for dinner.
W: That’s bad for health! Has she lost any weight?
M: She doesn’t look any thinner, but she hasn’t been on the diet long,
W: A friend of mine eats eggs for breakfast, lunch, and dinner. He never eats anything but eggs.
M: That’s bad for the heart ! Eggs have a lot of protein, and protein is good for losing weight. Too many eggs are bad for the heart.
W: I’ve read that, too. But eggs are cheaper than steak. An uncle of mine doesn’t eat anything  but steak, He eats steak for breakfast.
M: The best way to lose weight is to eat less and exercise more.