Okay, here's our theme:
If someone says to "Cheer up, don't be so depressing" to someone with a mental illness such as depression or anxiety… What's your reaction? What do you say?
It depends on the situation. If the person saying "cheer up" knew that the person was struggling with mental illness, I'd probably take a calm approach at first and say "hey, that's not how you're supposed to deal with things like this." And if they make up a bullshit argument like "you can get over depression" or "depression isn't real" or some stupid shit like that, I'd probably just tell them to fuck off and try to talk things out with the person with the mental illness and make sure they're doing okay.
However, if the person didn't know that they were struggling with a mental illness, but I did, I'd politely say "hey, that person might be going through a rough time right now, go easy on them." and if they refuse, I'd calmly ask them to leave so I could attempt to talk to the mentally ill person and make sure they're okay.