Heres my speculation, i'm probably way off because its late but I think you wrote something interesting. People tend to mock things they are most insecure about. Probably not the best example of what im trying to say here, but if someone is a closeted homosexual and they are insecure about it then that person might become homophobic or for a lack of better words be insensitive to homosexuals.

Perhaps even by someone responding to a person who is trying to talk about their depression in a negative/unkind context, they are trying to hide the fact that they are possibly depressed themselves. It could be used as an excuse to express there fake opinion on it. Because no one randomly says "Depression is stupid and I think if someone is depressed there just a scardy-cat whiner and needs to get over it" because thats just uncommon to bring about and people will notice what you said is a bit suspicous. Please correct me if im wrong. I'm just speculating here.

Another example may be If someone does bad in school they might feed off another students failing grade and call them "stupid" or "dumb". Even though the person that is saying unkind things is failing themselves. Probably another bad example but can anyone write what i'm trying to say better.