People tend to hold overly favorable views of their abilities in many social and intellectual domains... this overestimation occurs, in part, because people who are unskilled in these domains suffer a dual burden: Not only do these people reach erroneous conclusions and make unfortunate choices, but their incompetence robs them of the metacognitive ability to realize it. (Kruger and Dunning 2009.)
Well worth a read, ideally before you go around describing people with whom you disagree (and don't know) as being as thick as pigshit.
You might also bear in mind that social media amplifies this phenomenon.
edit: media amplifies