Why do people ‘accuse’ me of being on a diet?

It doesn’t really matter, but I just wonder why whenever people see me eating salad for lunch multiple days they ask me if I’m on a diet in an accusatory sort of tone? The fact that I eat salad because I like it is beside the point. So what if I was on a diet? Why is that anyone’s busy? Is there a rule book stating that only people over a certain weight are allowed to decide to eat healthier? Should I choose something greasier as not to offend anyone? Same sort of thing with exercising. If I do want to lose weight that’s my right and prerogative. I don’t see why it should offend anyone or cause judgment.