I typically don't have any issues with people who mean well, but it's interesting that phrases such as 'cheer up', 'just relax!' and similar things usually elicit a paradoxical response immediately, if not feelings of invalidation.
For example, once I was seeing a psychologist for anxiety (I didn't know I had ASD at the time) and something he said a lot was, "Just relax!". This made me even more aware of how I was projecting myself and basically made me withdrawal from him even more, because he couldn't handle normal human emotions.
So honestly, I wouldn't get irked if someone told me to cheer up, but they'd be invalidating my experience as a human, not looking for common ground and understanding, and I'd realize immediately that we're not emotional-equals and therefore I can definitely not confide in them ever again, only remaining cordial.
When someone can't deal with or recognize that humans go through the whole spectrum of emotions at varying times, you might as well be talking to a child.