I think it stems back to one of two things.......
Mama always said don't pick that scab! You'll get a scar. Soooooo naturally...... You know...... Gotta go against what mama says! Lol!
OR.........
It's just there. Something on your skin you subconsciously think skin should not have something on it so you pick it (Or brush it) away. It's not always there like a mole or something like that so if it just pops up you gotta get rid of it. Lol!
Or I could be over thinking this and it just itches so you scratch it off but THINK you have picked it off. Lololololol!!!