Do you really have to be overweight to be on a diet?

Diet. A diet is just a plan for eating, it doesn't have to be a weight loss diet. The best documented diet is probably the mediterranean diet.
No. People go on diets for different reasons. Most want to lose weight. Some feel that they may get more energy or feel better by being on a special diet. If your normal diet is full of saturated fats, sugary items and processed foods (and you aren't overweight), then i would say a different (and healthier) diet would be appropriate and even encouraged!
Diet is way. Of eating by establishing rules...Sometimes this got weight loss other times for good health in general.