It all started long ago, when the universe had that new-car smell and the gods were still wrinkling their noses and complaining of headaches. It was a tiny seed, tinier even than a seed of doubt. It planted itself in the heart of every god, and from it humanity was born.
Each human contains a small part of this seed, which is why each human is so unique and yet, in many respects, the same as every other. This tiny bit of the universe is born in our hearts alongside us, and as we grow up it's pumped through our body and it spreads thinner and thinner until it reaches our very extremities and we die. Not always literally, of course: the death of a human can be an extremely quiet, wholly unnoticed event. The word zombie should be used, in the gods' opinions, not for the dead who have risen but for the dead who never fall.
The seed of the universe manifests itself in different ways. For some, it's a tiny anxiety right in the center of their chest: it's always there, and they can never quite tell exactly why they feel the deep sadness they do. Those sorts of people are the ones who most often feel the deep navy blue embrace of the universe, because they are the ones who most often notice the empty sadness of the seed within them.
Others bury their sadness in meaningless tasks and empty words, drowning the seeds influence in a haze of invented goals. They invest heavily in the assigned importance of trivial things, and the saddest people look at them and wonder how they do it.
In the beginning, the universe was not an entirely sad place. But as with all things, as the universe grew, so its heartbeat spread the seed of its birth throughout its veins. And the gods scattered across the infinite galaxies, and the center of the universe was lost in its vastness.
The universe grew lonely.
Sometimes, in the endless night, the universe sang lullabies to itself, in the language the gods used to speak. The language of the nothing that came before, of the void without time. Over the years, as good a memory as it had, the universe forgot the lullabies and slowly went silent.
Thus is the inherent sadness of the universe.