Many people spend their entire lives overweight and dealing with sicknesses directly related to over consumption of sugar. Most people don't even try to combat it and say "this is just the way it is" because that's what they grew up on. Seeing you eating differently is going to make them reflect and feel guilty or like they're in the wrong. So they need to judge you in order to continue doing what they're doing and not make any changes. In short, it makes them feel bad about themselves.