Horry's life changed dramatically in one moment: the bar fight that left him epileptic. Like Paul's Rottweiler attack, Horry's bar fight is a seemingly small event with life-altering effects.
After the accident, Horry's relationship with Wendy breaks down. His dreams of starting a family are dashed. He becomes confined to his mother's home, which he resents but is unable to leave because of his condition.
The heartbreaking thing about Horry, however, isn't what he's lost—it's what he still has. He has "the kind of abs you want" (43.61). He's funny, caring, and never self-pitying. He even can still pick up attractive girls at bars.
But then his condition rears up and prevents him from enjoying these things to the fullest. A mid-intercourse seizure causes his female companion to run away terrified. He still has the opportunity to be close to Wendy at times, but he knows that they can never be together in the long run.
This Is Where I Leave You doesn't give Horry a fairy-tale ending—that's not the novel's style. Instead, it presents his struggles realistically and challenges us to think about how much life can change in a single moment.