Airplane loading

Why do passengers board the plane from the left side?

Traditionally, the captain of the aircraft sits in the cockpit on the left, so it is convenient for him to taxi to the airport building with the left side and watch the loading.
 
There is also an opinion that the embarkation of passengers from the left is connected with the history of navigation: in the Middle Ages, the steering paddle was attached to the stern on the right side, and the vessels were moored by the left side.
 
Modern liners have doors on both sides, but wide luggage hatches for the sake of useability are made at the right side of the airplane.

Therefore, for safety reasons, people are embarking from the left. 

This is how most terminals at airports are set up for.

Source: vokrugsveta.ru