A Thorn in Everyone's Sides
The West is a place so dangerous that even a tiny sliver of mesquite wood can kill a man. That wood is poisonous, and it almost kills Jake Spoon right off the bat. Lorena observes,
"It was bad luck, Jake having an accident so soon after they started, but it was just a thorn." (30.4)
The operative phrase here is "luck." Luck is a force that seems to rule Jake Spoon's life, mainly because he's a go-with-the-flow type who never takes any initiative to alter the flow's direction. Jake realizes this close to his death when he thinks,
"It seemed to him he had slid into bad luck in Arkansans the day he accidentally shot the dentist, and now he was about to slide out of it in Kanas and resume the kind of enjoyable life he felt he deserved." (71.5)
What entitles him to a happy life?
Jake doesn't realize that it isn't the luck, good or bad, that causes his problems: it's his reaction to it. And the mesquite thorn encapsulates this very well. Jake Spoon doesn't try to get it out. He lets it fester, and he waits for someone else to fix it for him. Deets has to get it out with a hot needle. If Deets hadn't come along, Jake probably would have died right then.
Call realizes this flaw in Jake's nature before he hangs Jake.
"It's a bad situation, but he put himself in it." (74.56)
And after Jake is gone, the thorn is all Lorena remembers:
"He had got a thorn in his hand, she remember that, but she didn't remember much else. She didn't much care that he was dead—he wasn't a good man, like Gus." (78.19)
In fact, Jake was like a mesquite thorn to Lorena, in a way. He got under her skin, and he infected her. His failure to protect her actually causes many of the bad things that befall her in the book.