When the negative feelings are described (interpreted!) as resulting from a supposedly pathological condition, for example. Also, when negative feelings result from the idea that a certain issue is pathological. And when you put these two together in a loop you have a pretty clear dynamic of pathologization, I'd say.

Of course it's hard to argue with the bare fact of someone feeling something, but everything surrounding that such as the attribution of the causes, potential solutions, and the terms in which feelings are described are all open to debate.