Telling someone with mental illness to just be normal is like telling a cancer patient to not have cancer, or telling a diabetic to not be diabetic.
It's something you HAVE and need to deal with and telling someone with the affliction to "cheer up" is really uninformed.
Agree, with a but.
Whilst you can't just wish it away, there ARE things you can do to fight it (analagous to chemo and radiation with cancer). You can get counseling and therapy. There are drugs which can help. You can learn coping mechanisms. You can build up friends and family to help you fight.
It's sort of a zen concept - you can't just "get over it", but you're not helpless either. You can fight it. You can get better.