I was recently asked if I thought depression and anxiety are common and increasing. My 'off the top of my head' answer was:
I think they are common. It certainly feels like they have become MORE common - but that just might be because more people talk about them and there's less Taboo.
They're a disease of the mind just like you can get diseases of the body. Chemical, intellectual or cultural imbalances.
As we (society overall) work our way up Maslow's Hierarchy of Needs people have more time to contemplate their own existence, less chance to exercise our Flight or Fight response in appropriate settings (e.g. running away from predators in the wild) and have access to more debilitating choices than ever before.
Also I think that a kind of 'survivors guilt' plays a part - where we know we are so blessed to have so much opportunity and yet others have so little and/or we haven't done enough with our gifts. At least I know I do.