Why do passengers board flights from left side?

Passengers board planes on the left side due to historical maritime norms. 

The tradition dates back to the days of ship sailing when passengers and freight were loaded on the left, or "port side." 

This legacy was carried over to aircraft, streamlining logistics and ensuring uniformity.

In the early days of aviation, most aircraft had propellers, which rotated clockwise. To avoid the propeller's swing, passengers boarded from the left side.

In case of an emergency evacuation, boarding from the left side allows passengers to exit the aircraft more quickly and safely, as the jet engine is located on the right side.