Economy class seats are usually quite small and no one wants to be stuck between two other people, sometimes there is no armrest because the two people next to you are occupying it.
Sitting in the middle seat, you have a hard time seeing or taking pictures of the sky outside the window. Many people have fallen into an awkward situation when trying to see the horizon because it makes the passenger sitting next to the window feel like you are staring at them.
The middle seat is also jammed by passengers sitting next to the aisle, which limits your ability to go to the toilet or stand up to put things in the overhead compartment.
