Why is that?

Sometimes the only reasonable and legitimate response to certain ideas and claims is to dismiss them. It's much the same for negativity. Sometimes a negative response is the best one.

Highly sanitized, politically correct, feel-good discussion is pretty much pointless to engage in. The best value and insight often comes from asking the hard questions and pointing out flaws that are painful to acknowledge.

It might not feel pleasant all of the time, and it might not make people happy, but engaging in discussion in a way that some people may consider "negative" or "dismissive" often gets the best results for everyone involved.