I always figured the head-tingling sensation while getting a hair cut was a hold over from our ancestral primates who needed to be groomed by one another and thus needed some pleasant sensory feedback to encourage seeking it out.
For me, it's more than just head tingling, it's a flood of utter joy and elation. It's an awesome experience when it happens. Quite where it originates from no one appears to know, and because it appears to be a good experience, no research is going into figuring out why it occurs (which is reasonable).
