On the very basic level, humans tend to look for meaning in existence. This is not something quantifiable. Philosophy, is that attempt to make sense of existence, truth, beauty or the search for wisdom.
Philosophical pursuit is done via a well reasoned argument. Every culture that advanced beyond the stone age has had theories of philosophy. These have been expounded upon by individuals and countered by other individuals. There has never been a consensus of thought for the reasons behind existence. I highly doubt there ever will be.
In reading about all the "isms" Rationalism, Empiricism, Skepticism, Idealism, Pragmatism and Existentialism. You realize this is all a spiritual pursuit. There are profound implications for how we could and should conduct ourselves, how we view and treat others. This enterprise can become quite absorbing.
It is a deeply rooted portion of our very nature. Why, do you suppose, we have such a deeply embedded need