Itches are contagious. Check out this simulation of an itch contagion from Temple University researcher Gil Yosipovitch. See if it makes you follow suit.
Why is it so easy to catch an itch from others? As Yosipovitch points out in an article written by Stephani Sutherland in the May Scientific American, there is an evolutionary basis for this mundane behavior.
The world of our Paleolithic ancestors was a pruritogenic wilderness, full of plants and bugs that posed more of a threat than mere skin irritation. So a scratch is a preemptive strike against the more noxious aftereffects if that relatively innocuous warning signal is ignored.
Update: A previous version misidentified the Scientific American article cited as the June issue