I view fat activism as a form of extremism. Extremists, whether only in how vocal they are or in their deeds, must project their doubts onto the world around them while supporting their own worldview.

As far as extremist movements go, the fat activism one is interesting, in that it is capable of existing independently of any political or social ideologies. You don't need third wave feminism or social fascism to help try and convince people that you are attractive, but it sure helps, at least in the minds of the fat activists.

From a purely psychological perspective these individuals are definitely projecting. Putting aside health, they know they're unattractive, but every person wants to be loved and valued; that creates a cognitive dissonance, especially since people are hardwired to view themselves in a positive light. People also have a problem with accepting that change exists: in their minds since you are fat now you will always be fat. This is why they suggest that weight is not a chosen trait.

Extremists also like conspiracy theories. If you can't convince someone rationally, convince them emotionally, and what could elicit an emotional response more than the idea that someone is being controlled? But if I'm being controlled by some white male media conglomerate, wouldn't advertising to me that fat is healthy and sexy just be more of the same?

And if left to my own devices, why do I come to the same conclusion: that I am attracted to petite athletic women? The answer of course is that there is no conspiracy, and my own biology tells me that, even if I'm only having sex for fun, an unhealthy partner like a fat person is a bad idea, no matter how much I enjoy their personality. All of these realities combined create an extremist fat activist.