My gut feeling (and this is completely not scientific) is that Hacker News tends to have a large percentage of people who are contrarian... which kind of makes some sense since if you were just following the crowd then you'd probably not be on HN, or possibly not even a hacker.

But the end result is that people take a lot more pleasure in showing that they can debunk or argument against something. Saying: "I totally agree" may feel a bit like you are following someone else's thought, and most people here take pride in thinking differently.

I also think patio (HN values cleverness) is another important factor.