You ruin everything, Patrick.
Researchers theorize that this is because we tend to focus primarily on the plot and the resulting suspense, which prevents us from chilling out and taking in all the other wondrous elements that make up good storytelling. Or, as Jonathan Leavitt of UC San Diego's psychology department more eloquently put it, "Once you know how it turns out, it's cognitively easier -- you're more comfortable processing the information -- and [you] can focus on a deeper understanding of the story."
Everyone has that one book they can read again and again, or that one movie that only gets better with repeated viewings. If spoilers really were as big of a deal as we make them out to be, wouldn't it stand to reason that your enjoyment would decrease with each subsequent viewing?
Everything gets worse. Everything.
So the next time some irate Internet denizen yells at you for revealing a spoiler, you just tell them that, scientifically speaking, you're actually doing them a favor. They'll have no cause to type homophobic slurs at you now!