Leaving chemical depression aside, because who knows what environmental crap is going on to make a mass of chemical depression. We have two general options for depression en mass as it has been seen in recent years.
1) The sociocultural senses of the definitions of happy and sad have changed. Are more people depressed because more people are depressed or are more people depressed because we think that happiness, and dealing with depression are different than people did 50 years ago. The same argument has been made about several developmental problems like ADHD, autism, dyslexia etc... the argument boils down to, we treat what once were considered "stupid and/or mopey" people differently now.
2) People have more time to be depressed, and/or its vogue to be depressed. Young minds need constant stimulation, (old minds as well but not to such a developmental extent) yet people naturally follow the path of least resistence. So idle people have lots of time to, as Ergo said, live in their heads. Living in one's head without some real sense of meditative direction deprives the body of the activity and the chemical beinfits of the activity it needs to maintain good function and chemical balance. It also gives people time to reflect on that which they do not have and are not doing and are not accomplishing. This breeds a cycle of dissapointed expectations cultural and personal which in turn creates anxiety. Whole affluent nations of bored, lazy dissafected young people have built a poplular culture that celebrates affluent depression, and celebrity that gets "easy money". Cycling, i could be happy if, but i won't be happy because.
And now I'm just rambling