In my case, it was due to several factors, years apart from each other:
In 1970, when I was a freshman in high school, my civics teacher required a
research paper on any area in which government provides some type of necessary
service to the public. Given that BART had just begun operation (or was about
to, I forget), I chose mass transit, and even managed to include a sneak
preview of how my city's (Ventura, California) transit agency was about to
restructure and expand service. (I guess they couldn't resist telling an
interested kid about their plans, even though they were more than a year away
for implementation.)
Several years later, Ventura's transit agency merged with the Oxnard/Port
Hueneme joint-operation to form South Coast Area Transit, right about the time
that I found myself in a position both to use and report on mass transit (I
was in radio by then). Needless to say, I was probably the most knowledgeable
passenger on SCAT in their early years. :)
And at the end of 1991, when my car was damaged beyond repair in an accident,
forcing me onto RTD, old habits came into play and my mind started cataloging
every bit of information possible. This has come in quite handy, though,
since it brought me into contact with MTA as the San Fernando Valley Transit
Restructuring Plan was completed (and my contacts at the agency have increased
substantially since then, with my letters to the editor, public comments at
board meetings and hearings, and my service as a passenger advisor).