In my experience, the inability to be satisfied with a materialistic world-view comes down to simple ego preservation, meaning, fear of death and the annihilation of our selves. The idea that everything we are and have ever known will be wiped out without a trace is literally inconceivable to many. The one common factor in all religions or spiritual ideologies is some sort of preservation of 'soul', whether it be a fully platonic heaven like the Christian belief, a more material resurrection like the Jewish idea, or more abstract ideas found in Eastern and New Age ideologies. The root of spiritual, 'spirit', is a non-corporeal substance/entity whose main purpose is to contrast itself with the material body. Spirit is that which is not material and so can survive the loss of material pattern decay.

In my opinion, THIS IS the hard pill to swallow.