We're reluctant to maintain random automobiles because there are so many unknowns. We'd prefer to develop a relationship with the automobile and know it over time. There must be a log which includes such vitals as when the last valve adjustment was performed. Further, once charged with performing maintenance on an automobile, we don't like it when we share this duty with another mechanic yet, given the distances involved today, it can be a real challenge to have ready access when a car needs assistance. Finally, we're very adverse to being blamed for whatever might go wrong - for example, just because we might have performed a tune up there's probably not a causal relationship to the unfortunate circumstance that three days later your engine had no oil pressure and died. No, it was probably the other guy, but people tend to blame whoever touched it last.
...That said, for local people who are lucky enough to have a quality machine for us to attend to, we'll maintain your vehicle!
OK, so for you lucky ones, you'll need a logbook and convince us you actually use it. So if you haven't got one for your automobile already, get out there and start one today!