I have a friend who is a pilot and although freight and catering can be loaded from both sides it is normally loaded on the right for passenger aircraft. This keeps passengers away from support vehicles when being boarded from the ground and avoids accidents and security concerns.
I can only guess that if all pasenger aircraft load on the left you would look a bit silly turning up at a major airport opening the wrong door!
they also [when an aircraft is doing a quick turn around] re-cater/empty bins while folk are embarking or boarding so it makes sense to keep 'em away from open doors as the tarmac is quite hard.