Why does one school of thought always endeavor to invalidate another?

On Facebook, I just saw a pic of a Bible with the caption “Sin is an imaginary disease to sell you a imaginary cure.”  Facebook friend Vanessa Wells wrote “Love the quote. Even A Course In Miracles says this is true. But why does one school of thought always endeavor to invalidate another school of thought, i.e., Humanism vs. Faith and Mysticism? Why do we insist on living in an “either-or” type of reality. What’s the drive behind our need to be “Right”, and therefore, logically, that means that you are “Wrong”. It’s has to be that sacred belief in scarcity, that there’s not enough world for everybody to be right. Crazy.”