Suggestion is a powerful thing. When I was 14 and I worked as a grocery store cashier, I once decided I couldn't stand being there that day and was preparing to say I was sick and needed to go home. What I didn't realize was that as I was ringing in orders and mentally rehearsing what I would say to my manager when I saw her ("I'm not feeling well..." "I have a terrible headache..." "I've never felt so awful in my life..." etc.) I was actually convincing
myself that I was sick and by the time I left work, I truly wasn't feeling well at all.
It's funny how the mind works. Supposedly you can use this sort of self suggestion in a positive way too. All I know is, it's strange what you find yourself saying if you actually stop and listen.