Spamcanner, the original poster, and others are correct in that you mount your aircraft from the left because that's the side you mount a horse - although as with every rule there are exceptions. Civil aircraft still have entrance doors on the left (that connect with the bridges at airports) and the right side doors are service doors.
As has been said the ability to ride a horse was seen in the early days of aviation as an indicator of flying ability and thus it was seen as normal to enter your aircraft from the left hand side. As aircraft became more sophisticated it made sense to standardize on entry/ exit on the left & refueling & other services on the right so that they didn't interfere with each other.
But as I said, there are always exceptions - Lancaster, Mosquito, B17 amongst others. A number of light aircraft also have the door on the right so as to allow entry/exit without interfering with the person in the pilot-in-command seat.