When people interject positive posts with venom and negativity.

Person talks about something positive and there is always that one person that cannot wait to sour the whole thing. Why? If any of you are in the audience, what do you get out of this? I know misery loves company but why is that? What makes people like this where they see someone or a group of people who are happy and having a good time and they just want to bring it all down. I seriously don't get it.