There are three reasons. First, an excessive concern about one's security. Next, what one does not know always gives an uneasy feeling which is translated in the consciousness by fear. And above all, one doesn't have the habit of a spontaneous trust in the Divine.
Because in the large majority of men, the body receives its inspirations from the subconscient, it is under the influence of the subconscient. All the fears driven out from the active consciousness go and take refuge there and then, naturally, they have to be chased out from the subconscient and uprooted from there. <ref>http://incarnateword.in/cwm/04/14-march-1951#p27</ref>