Character Analysis
Louise's best friend in the entire world is Call. Of course, even though this kid is pretty much the only person she hangs out with, she doesn't really seem to like him much:
Call was a year older than I and would never have gone crabbing with a girl except that his father was dead, so he had no man to take him on board a regular crab boat. He was, as well, a boy who had matured slowly, and being fat and nearsighted, he was dismissed by most of the island boys.
Call and I made quite a pair […] He, at fourteen, was pudgy, bespectacled, and totally unsentimental. (1.1-2)
Yeah, she doesn't think much of her BFF; his sense of humor really annoys her, too, and later she says that Call is a "second-rate person" (12.41). Bummer. Even given all of this, though, Louise doesn't want to share Call with anyone. Not the Captain, not her father, and especially not Caroline.
Call hasn't exactly had the easiest life. He has no father and his family—made up of his mom and grandma—is pretty darn poor. He doesn't have many friend options either, which might be why he and Louise are drawn to each other. Call desperately needs a male role model in his life, so it's nice that he can help Louise's father on his boat and earn a little extra money for his family. Louise isn't so ticked about this—she kind of gets it—except that it means she has no one to hang out with.
Throughout the novel, Louise sort of stays the same, but Call grows into a man in his own right and changes a whole lot. After he comes back from serving in the war, chubby, awkward Call has turned into a mega hunk:
He stood there in his petty officer's uniform, tall and almost shockingly broad-shouldered and thin-hipped, his cap pushed slightly back, the sun lighting on the patch of reddish hair that showed. His eyes were bright blue and smiling down at me, and his nose had mysteriously shrunk to fit his face. I realized that I was staring at him and that he was enjoying it. I looked away, embarrassed. (16.37)
Suddenly, Louise has eyes for Call. Sure, she always cared about her best friend, but she couldn't be bothered to see him as anything special. Call even points this out; he knows Louise never thought much of him. She clearly doesn't think he deserves Caroline, so why would he want to be with her? All she's ever given him is sass and a crabby attitude.
That's why it makes a lot of sense that Caroline and Call hook up. After Louise bails on their friend group (since she can't stand sharing Call and the Captain with Caroline), Caroline and Call spend a lot of time together at the Captain's house. While Louise seems mostly peeved with Call, Caroline dotes on him and makes him feel special and loved. Why wouldn't Call want to marry the sweet, beautiful, loving sister? Sorry, Louise—you dug your own hole with this one.
Plus, as Call points out, Caroline needs him, too. For Caroline, Call is a connection to home. He's grown up to be someone who can take care of her while she's out achieving her dreams. In other words, she needs Call in order to truly make it out there in the big city. Louise would never need the same from Call—it's just not who she is—and she's made this clear since they were just kids playing on the island.