I think the comments about having something unhealthy or loosening up is maybe a number of causes. Jealousy, like someone said, they kind of don't want to say oh well done as they fear the other person may take that as a sign they need to lose weight.
But for me personally. And I've been eating healthy and exercising every day for months. I've cut out sugar, most carbs (they are my downfall), all the crap so I'm not jealous..... but from talking to other people I think there is a bit of a backlash with the whole clean eating/paleo/I'm going to put a pic up of my clean meal every single day thing that has gone crazy the last few years. There has been such a rise in this obsessive, Orthorexic type lifestyle, Freelee the Banana Girl is a good example.
I'm not saying anyone in this thread that has felt judged for being healthy fits this mold. I'm just speaking generally that a lot of people are sick of the fitness craze.