What pleasure do you get out of wrecking someone else's surpise?

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I was in a restaurant having coffee this morning and my friend whispered to me that she is throwing a surprise party for another friend of ours in a few weeks. She said she wanted to do it last year but someone told the person and ruined the surprise. Why do people want to do mean things like this? I just don't understand why ruining other people's happiness brings some people pleasure.