Some psychology student somewhere will google this title and be a little confused as to what's it's doing here. I suppose most other people may be confused too.
As a psychology student, one of the many (many, many) things I have learnt about this last year is the fundamental attribution error. Generally, this is the belief (often made erroneously) that a person's behaviour is due to their own opinions and motivations.
For example, there was a psychology study in which participants were asked to listen to two opposing speeches. Both of the people giving the speeches were given a particular topic to speak on that was controversial and didn't necessarily fit with their viewpoint. Despite the participants being told that the speech topic had nothing to do with the speaker's views, they still attributed the opinions in the speech to the person speaking. So the speaker talking about how battery farming was great was viewed as being someone who supported battery farming, despite always buying free range.
This can also apply to everyday things, such as assuming that a sharp reply from someone is because they dislike you maybe, or have little patience. Usually, this isn't the case (hence F. A. error). I'm not sure why people see others like this but it has some negative consequences when people see the behaviour of those with invisible illnesses.
If I was chatting with a friend and then had to say that I was sorry but I needed to go rest, this could very easily be perceived as me being bored with the conversation rather than me being exhausted and having to drag myself away. And it often is perceived that way. Unfortunately in my world, and in the world of many others with invisible illnesses, can't and won't are entirely different. People who have had activities taken away from them due to illness generally would LOVE to keep doing them. Even doctors sometimes get this one wrong and see the patient's describing being unable to do things as not wanting to do things and diagnose it as depression.
Our brains would like to fit things into neat boxes and so will attempt to make the fundamental attribution error, but you don't have to listen.