Why does an airplane captain sit in the left seat?
There are several reasons, mostly historical, or theories behind this:


After World War I, most airplanes had rotary engines with left turning tendencies because it followed the torque of the engine. Therefore, turning left was easier than turning right. Because of this, pilots considered left turns as more convenient maneuver, and thus more experienced pilot started sitting on the left.
Since the early days of aviation, the fact that pilot was sitting on the left, they keep the airplane on the right side along the airway. At that time, pilots often navigate visually following roads and railways. Opposite traffic following the same path then pass each other on the left.
Because captain sits on the left, left-hand traffic patterns are more common.
In order to avoid a head-on collision, each airplane must turn right. Since the captain sits on the left, they will have an unrestricted view of the other airplane, and therefore can assess the situation easily.


In modern times, almost all airplanes (other than single-pilot airplanes or where one pilot sits behind the other), both seats have full controls to fly the airplane.

Regulations do not specify which seat should be occupied by the pilot-in-command (PIC) or the Pilot Flying (PF).

Please note that on many helicopters, captains sit on the right side.



Bibliography:


Why a Captain sits on the left side of a plane?
Why Does The Captain Sit On The Left? 
Why Captain sits on the Left?