Truth be told, I first come to know about Cognitive Dissonance while I was browsing Reddit. From there I learned that people who is suffering from this mental discomfort is mostly caused by their strong belief and faith in something. 

Take for example people who believe that the earth is flat. Even if you present scientific facts and evidence that disproves their belief, they will not accept it and will try to rationalize, ignore and even blatantly deny those facts. 

Because their belief is so strong that they feel the need to protect it. Anything that goes against their belief will be ignored. Over time this will likely cause a contradictory belief and thus physchologically stress them all out.