Word choice aside, I do see what Stephy is saying.
There are people I've met who, not through some amazing coping skill, but more of some kind of learned avoidance, completely and utterly dodge acknowledging any kind of negativity in the world.
It's not the cheerfulness itself that bothers me. Overall, I'm a pretty cheerful and content person. It's the "LA LA LA, I CAN'T SEE THAT" that's bothersome. Happy or not, some aspects of the world needs to be acknowledged, and totally ignoring its existence isn't particularly healthy.
Encouraging or not, people like to feel like their trials are being acknowledged rather than have them minimized by someone who who's like, "Fa la la, you should just be happy like me, because bad things don't exist an here's a peppy quote." Every. Single. Time.
Yes it's wonderful (and usually helpful) to be encouraging. Sometimes people don't want or need a pep talk. Sometimes it's not appropriate. Sometimes all people need is for someone to say, "hey man, I'm sorry you're going through such a tough time." And it is possible to be encouraging while still being sympathetic.
I do think it's good to be positive. I do think it's healthy to have a positive outlook. I don't think it's healthy for people to put their fingers in their ears and pretend sad things don't happen or exist. It might be a kind of coping skill, but it's not necessarily the healthiest, and I do understand how that can make someone look oblivious in either a deliberate or a "dumb" way. People are built to feel a range of emotions, and being unable or unwilling to get out of one extreme and feel the other doesn't strike me as healthy whether it's happy or sad.