One mystery to me, personally, is that the average person does not understand that art is essential for humans. I'm not surprised that people don't understand WHY art is so important -- indeed, I myself do not really -- but simply that people think art is a frivolous option, something to while away our free moments.
To keep this from being too much of a cornpone idea, I'll use music as my artistic example, rather than literature.
Every culture has music. This is not to say that most cultures have music, or that nearly every culture except for some small group in the rainforest or in the distant past has had music. What I mean to say is at the same time audacious, undeniable, and painfully obvious: every culture has music (and other art). The specifics of the types of music and instruments differ from culture to culture, yet all still have music. Humanity does not seem able to exist without music.
Even the deaf enjoy music. Of course, everyone knows of Beethoven's compositions after going deaf, but some might argue that he was hearing the music in his mind's ear, which is likely enough to be true. The counter-example is that people who have been profoundly deaf from birth also enjoy music; deaf teens, for example, will sometimes hold dances with the music very loud, and the bassline very strong, so that they can "hear" the music through the vibrations in the floor. I've seen one such dance, and the power of the bass in the air rattled my bones and sent me quickly out the door.
In fact, while humans cannot seem to exist in groups without music, they are not the only such animal. Take neanderthals, for example, who we know made musical instruments. Neanderthals are not only not true homo sapiens, but some recent genetic research suggests that they are not even very closely related to us (with a tree dweller being the closest common ancestor). Their oral structure made human-level speech impossible. Yet they too made music. I like to imagine that they had little bands of bone flute players.
Steven Pinker claims that music is an evolutionary accident that does not promote the survival the the species, famously calling it "auditory cheesecake." And yet we can't seem to survive without it. Music seems to be in the category of sex for survivability, i.e. individuals can survive without having sex, but the species cannot. Individuals can survive without music, but the species cannot. Why they cannot, I do not really understand, yet there it is.
Obviously, I'm trying to make the same claim for other forms of art (such as literature). Every culture has poetry, for example, or visual art such as painting or sculpture. We ignore the importance of art at our own peril.