Passengers typically board planes from the left side for several reasons, including historical, practical, and safety considerations. Here are a few factors that contribute to this practice:

1. Historical Reasons: The tradition of boarding from the left side dates back to early aviation when aircraft were often parked on the grass or unpaved surfaces. Pilots used to approach the aircraft from the left side, which allowed them to use their right hand to greet and assist passengers while their left hand was free to hold on to the aircraft’s structure.
2. Standardization: Boarding from the left side has become a standardized practice in the aviation industry. This uniformity simplifies processes, reduces confusion, and ensures efficient boarding operations for both passengers and airline staff.
3. Safety Considerations: In many airports, the infrastructure and facilities, such as boarding gates, jet bridges, and equipment, are designed to facilitate boarding from the left side. This consistency helps streamline the boarding process and reduces the chances of confusion or accidents that could arise from attempting to board from different sides.
4. Separation of Passenger and Baggage Loading: Boarding from the left side allows passengers to enter the aircraft while the ground crew simultaneously loads baggage and cargo onto the right side of the aircraft. This separation of passenger and cargo operations helps maintain safety and efficiency during the boarding process.

It’s worth noting that while boarding from the left side is the general practice, there can be exceptions or variations depending on specific circumstances. For example, in some cases, when boarding stairs or airstairs are used instead of jet bridges, passengers may board from the front or rear doors on either side of the aircraft.

Ultimately, the specific boarding procedures and practices may vary slightly between airlines and airports, but the industry-wide adoption of boarding from the left side helps ensure consistency, efficiency, and safety in the boarding process.