Mental illness is ultimately a personal issue that affects everyone very differently. You get a picture of this with how the same SSRI or other medication can have wildly different responses in people.

I wouldn't be so flippant and dismissive of terms like PTSD and Depression. For the person suffering them they are going to be very real and different than everyday sadness or misery. Even if for us outsiders they don't meet some documented guidelines of what they are.