I've watched the Harry Potter films many times over the years, and each time I always felt a little confused as to why Hermione Granger ended up with Ron Weasley. Throughout the series, Ron is (in Harry's words) a right foul git to Hermione.
 Sorcerer's Stone: After the students practice the Levitation Charm (Wingardium Leviosa) in Charms class, Ron openly mocks Hermione to his classmates. Hermoine happens to overhear the cruel conversation and runs by, crying.
 Prisoner of Azkaban: Near the beginning of the movie, Ron is quarreling with Hermione about her pet cat, Crookshanks, and makes vicious comments.
 Prisoner of Azkaban: Later in the film, Ron makes additional nasty comments about Hermione's pet.
 Prisoner of Azkaban: After Hagrid reveals that Ron's pet rat, Scabbers, is alive, Hermione attempts to elicit an apology out of Ron for his earlier comments. Instead of contriteness, Ron offers a disingenuous apology toward her cat to spite her request.