People can be hypochondriac to mental illness as well as physical ones. It is that nasty "What if someone missed something?", there is always degrees of unknown. Accepting the unknown is hard. We need to sort of be oblivious to most kinds of fears when you think of it, or we would think of it all day.

Going from not thinking about something to thinking about it is rather easy, but it is quite hard to tuck thoughts back into the unconscious. Sometimes distractions can help.