We tend to defend vigorously things that in our hearts we are not certain about. If we are certain of something we know, it doesn’t need defending. ~ Madeleine L’Engle
In a way the above quote is true as people seldom see the need to defend something or some view they are certain about. For example, a philosophical question goes like “How do you know the sun will rise tomorrow?” I would say this is a certain fact and many people would not think much about it except for academic exercise, using inductive approach to answer the question, such as “The sun has always risen every day in the past, therefore I know the sun will rise tomorrow.”
When it comes to things that are relatively unknown, such as metaphysics or religion and spirituality, and things that are unpredictable, such as weather forecast or outcome of a sports match, people are usually uncertain about these things and have to formulate theories to explain them. Maybe some people get too attached to their theories or ideology to the extent that they feel they have to defend it passionately. We see that especially in the religious circles.