Like Ann, Mary seems like she's out of Abe's league. (If anything, this is a book that tells us that love conquers all, except for vampires.) Mary comes from a wealthier family and when they meet in 1839, Abe is a lawyer and has won one state election.
But they're drawn to each other, in part at least because they're both smart cookies. Also, they both lost their mothers and are moody mopes, prone to depression (7.72). From this description, it's clear that Abe has a type that both Ann and Mary fit into: he likes smart women.
And Mary has a type she likes, too: smart, funny men, like Abe. Sure, maybe Abe doesn't have a lot of money, but, "In the end [... she] decided that it was more important to laugh than eat" (7.74). And that helps show us a part of Abe that we don't always see while he's killing vampires: Abe is funny. Who knew?
Unfortunately, Abe doesn't seem to trust his loves enough to tell them about the vampire threat they face, which we think is a bit of a problem. So Mary's later life is full of paranoia about strange men following her and seeing Abe's face in the darkness, which we can probably assume are vampires and Abe chasing after them (14.24). (Or possibly she's just a little crazy in this book, as she kind of was in real life. Although, we can't really blame her for being a little on edge after her husband gets shot in the head right next to her.)
Imagine how much easier her later years would've been if Abe had only told her about vampires. Then, at least, she would know that she's not crazy, but being shadowed by supernatural creatures that feed off of blood. That's much more relaxing. She could have really let loose.
By looking at the loves in Abe's life, we can see a pattern. He doesn't share the secret of the vampires, which might make his life easier. Instead, he keeps that a secret from the closest people in his life and takes (or tries to take) all the punishment on himself.
Hmmm, whom does that sound like? We think that sounds a bit like his father, Thomas Lincoln. Instead of asking for help when Thomas owes money to the vampire Jack Barts, Thomas doesn't say anything to anyone—not about the money he owes or the fact that the guy he owes it to is a vampire. Of course that comes back to bite him in the butt in the end, so all we can do is hope that pattern doesn't repeat itself with his son.
Except it does. Abe loses Ann because he fails to see the danger of the vampire John MacNamar and writes a letter, foolishly. And Abe loses Mary (for a while) when he learns that her dad trades slaves to vampires. But instead of using his words, which in this case would actually be a good idea, Abe runs from Mary and stays mum about why. Besides showing us that Abe can screw up royally, Abe's mistakes with Ann and Mary also show us that (a) words may not be enough and sometimes you need an ax (in Ann's case), but (b) that doesn't mean you can get away without using any words.
In fact, put that way, Abe's next move makes a lot of sense: Speed and Abe invite a bunch of vampires to come to Speed's house (words) and then Abe chops their heads off (ax). Finally, it seems like Abe is learning his lessons that will help him win the Civil War: words and axes are better together, like peanut butter and jelly.