Why do planes board from the left?

This is the first in the series, One Question Leads to Another, and is part of the @Warmbru Curiosity channel. This series will look at many things in life that we take for granted but often have an interesting explanation. In this episode, we ask why we board aircraft from the left. The answer takes us back into the past.
Link to the 'Lions Led By Donkeys.' myth.
https://www.youtube.com/watch?v=6vOfiBLZamY