All his life, Taehyung has had bitter blood, and not by choice. It was just that he was born this way, and he's always suffered for it.
In a world where vampires and humans now coexist along side each other, blood is extremely valuable. Since people are going to treasure them (and since a vampire's main food source is blood, of course), tons of people offer blood. There are also 'mates', which the supernatural beings have. It works in the same way as soulmates, and love at first sight, except the vampires are affected particularly much more, in the sense of romance and lust.
With the horrible treatment Taehyung received all his life, the one thing that's keeping him on the ground is the potential mate that he knows he'll meet. And so, he meets Jungkook.