My first thought is that you could be a highly sensitive individual. The world, especially right now, is a pretty tough place to be.
We've all been through so much in the last 6 years; it's been kind of mind-blowing for me, at times. If one is intelligent and paying attention to all the madness going on, it's no wonder that garden variety sadness can linger like it does.
I find myself feeling this way now, too---although my Major Depression symptoms have lifted a great deal, and I only struggle, on average, through 3-4 tough days, about once a month.