I think you mean, "herd," as in "a large group of animals that live, feed, or migrate together," not "heard," as in "I heard what you were saying, and I disagree."

But it's a serious question.  Let's say that I don't like Bob Dylan, for instance.  What would motivate me to join a forum of Bob Dylan fans and spend hours and hours telling everyone that Bob Dylan has a whiny, nasal voice, and that he sings off key?  Clearly true, but still, why devote big sections of your life to seeking out disagreement?

You're suggesting that choosing to hang out with like-minded people makes me a herd animal, but I'm wondering if devoting ones life to going to Yankees ball games just to tell the Yankees fans how terrible the Yankees are isn't a terrible waste of time and energy.

My brother-in-law likes old cars, and he goes to "Cars and Coffee" every Saturday morning.  Imagine that some person started showing up every week to tell everyone there that old cars are stupid and worthless.  I would wonder if that person had some kind of mental illness that made him go out of his way to be so disagreeable!