Most airline regulations have a safety purpose behind them. Safety is the reason airplane seats must be upright for takeoff and landing.
A primary purpose for upright airplane seats is ease of egress, or exiting, from the plane. In a non-emergency situation, deplaning would be a much longer and more inconvenient process if passengers had to nearly climb over seat backs in order to get to the aisles of the aircraft. The economy classes in most aircraft have an average seat pitch (the space between seats in a row) of about 31 inches (78.7 centimeters). This is a tight fit with upright seats. It is easy to imagine how small the space becomes when a seat back intrudes into the already narrow area.
In emergency situations, clear access to the aircraft aisles is imperative. Passengers must be able to reach the emergency exits as quickly as possible. Since most aircraft emergencies occur during takeoff or landing, having the airplane seats up at this time is crucial. It also allows passengers to assume the "crash position" if necessary.
Another reason for upright airplane seats is that the seats can become weapons in a crash, or even in a hard landing. A reclined seat back could kill or seriously injure the passenger behind, if it should come unbolted, or if the passenger behind it is thrown forward. So, upright airplane seats are necessary.
Most aircraft are airborne in just a few seconds after beginning the takeoff roll. Final approach usually takes only a few minutes. It is not a great inconvenience to have one's seat upright for those few minutes of the flight. When passengers consider safety of the other passengers first, and their own comfort second, they will not complain about the upright airplane seats rule. They will understand it helps contribute to the safety of all on board the aircraft.