I (and tuners) have seen it time and time again. "I just wan to be able to tune my own car" eventually leads to tuning for friends for free (cut throat competition) and then Friends of friends for a fraction of a tuner's price (cut throat competition) and on and on.
I've personally spent more than my share of time training people who turned around and competed against me. And I've done the same to others.
Nobody ever learns a skill to exclusively help themselves.