Have you ever wondered why you have to return your seat to 'an upright position' before your flight takes off or lands?
The reason has to do with evacuating the airplane safely in case of an emergency.
See also: How to combat your fear of turbulence
If you're sitting behind a reclined seat, it can prevent you from getting to the aisle as quickly as possible.
The majority of accidents occur during takeoff or landing, hence it being mandatory during these parts of the flight.
Putting your seat back up can be annoying, but it's a small price to pay for what could be a life-saving act.