Why do passengers board, alight an aircraft only from the left?

The hold doors are on the right, so with the baggage being loaded/unloaded and re-fuelling taking place, it would be pretty crowded and dangerous. Also on older aircraft, the forward right hand doors were actually designated service doors and emergency exits only. Besides, the left side of the aircraft is where the pilot sits, with the co-pilot in the right-hand seat. In big new planes, the pilot has no visibility on the right. Taxiing to a gateway via the right side allows the pilot to see with more accuracy exactly where to stop, though the gangways can be moved.