This is basic human behavior. Humans, for the most part, are social animals. That means we identify ourselves by forming a group of like-minded people whom we share ideas with, as opposed to different-minded people. If different-minded people form a big enough group, there is a chance that some of our like-minded people will turn tail on us (due to mob behavior or social factors) because the now bigger group has more like-minded ideas than ours, even if a couple of primary ones differ.
This behavior is necessary for "functioning" societies to work. The other option is to change people's minds by expanding their thought. But this process takes a very long time, since most people aren't interested in changing their worldview overnight.