Pride and Prejudice
Mr. Wickham and Mr. Darcy
For a large portion of the book, Elizabeth directs her hate towards Mr. Darcy. His cold, haughty demeanor is such a contrast to her own playful, lively one that she has no trouble believing the worst of him (plus, he insulted her looks). Obviously, Darcy doesn't remain the novel's antagonist for too long. Soon that title switches to Wickham. Once Elizabeth starts to have feelings for Mr. Darcy, Mr. Wickham gets in the way, and she starts worrying that Wickham's seduction of Lydia will drive Mr. Darcy away for good. Dun dun dun.
For most of the book, Mr. Darcy stands in Lizzy's way: he separates Jane and Bingley, and he (allegedly) destroys Wickham's life. Thanks to his cold, haughty demeanor, we (and everyone else) have no trouble believing the worst of him.
One last possibility: when you think about it, it's really the Bennet family that gets in the way of Elizabeth's happily ever after. Her idiot mother, flirtatious sisters, and neglectful father nearly ruin her life for good. It's a good thing that Pemberley isn't very close to Longbourn.