Final answer:
A passenger may be required to purchase a second seat on an airplane primarily due to their size for comfort and safety reasons. Other constraints in airline travel include security measures and efficiency in operations.
Step-by-step explanation:
We require a passenger to purchase a second seat on an airplane primarily due to the passenger's size. When a passenger cannot comfortably fit within the confines of a single seat or when the seatbelt does not adequately span the passenger's body, it is a standard airline policy to require the purchase of an additional seat.
Airline travel does face numerous other constraints such as increased security screening, efficiency considerations as seen in the hub-and-spoke system, and the importance of adequate safety equipment for all passengers including babies.