"The boundaries which divide Life from Death are at best shadowy and vague. Who shall say where the one ends, and where the other begins?" —Edgar Allan Poe
Before we even know where this quote is from, this epigraph makes sense for two reasons:
Plus, this line is from Poe's short story, "The Premature Burial" (1844), which gives us a third reason. That story is about a guy who is obsessed with the idea of being buried alive; and so he spends all his time worrying about death-by-dirt rather than enjoying life.
Does that remind you of Abraham Lincoln? Sure, Abe marries and has kids and lives a full life. But his whole life is shadowed by this obsession with vampires, just like the narrator of "The Premature Burial" lives his life shadowed by the fear of being buried alive.
When Abe fights with Henry, he says something that reminds us of this: "The whole of my life has been spent on your errands, Henry! The whole of my life! And to what end? To what happiness of my own? All that I have ever loved has fallen prey to your kind! I have given you everything" (11.112).
So here's Abe, trying to go through his life; but everything in his life is sacrificed to his mission of killing vampires. In the same way, the narrator of Poe's "The Premature Burial" is so obsessed with his "mission" (not being buried alive) that he sacrifices his whole life to that mission.
And (for a fourth reason, if you need it) it's a nice little reminder that Edgar Allan Poe lived at the same time as Lincoln, so when he shows up as a character, we're not so surprised.