The pilot sits in the left seat, by convention/tradition.
Early airports were set up so that aircraft could taxi in front of the terminal and stop to discharge passengers. It was useful for the pilot to be able to judge wing clearance from the terminal building and to put the aircraft door in front of the terminal doors. Some early transports had right-side doors into the passenger cabin, but the logic of the pilot's field of view prevailed. The pattern became fixed over time, and as airports and aircraft handling became more complex, it has continued.
Safety regulations mandate a ratio of doors to passengers, so that large civil aircraft will have four passenger doors (two forward, two at the tail, and two or four smaller exits over the wings. At some airports, stairs may be placed at forward and/or aft doors, but to avoid passengers wandering around a busy parking ramp, the loading/unloading is always on the same (almost always the aircraft's left) side. Also for safety reasons, passenger movement is kept on one side while fueling is carried out on the other. It is also useful to unload/load baggage and cargo on the opposite side from passenger movement and the loading bridge or stairs.
[EDIT ADDED 3/31/2015]
A duplicate question, redirected into this question, also asked why baggage couldn't be unloaded from the left. Originally, aircraft baggage compartments did open on the left, and passengers and bags were handled on the ramp as part of the embarkation/disembarking process. As aircraft grew and more passengers were involved, it became important not to have a planeload of people milling around on the ramp, seeking to put their bags on the top (for LIFO access) or waiting to get their bags. Cargo hatches have become large openings in the aircraft structure, and having such hatches on both sides weakens the structure and complicates placement of bags in the hold. The opposite-to-embarkation-side placement and right-side ground-handling convention became virtues in the design of hatch placement in structures, avoiding the drawbacks of having emergency passenger exits on both sides. [END EDIT]