Time and time again my partner and I have flown on domestic flights in the window and middle seats, and found ourselves with a person placed (more) comfortably on the isle seat next to us. Look up and all the rows in front of us have spare middle seats. Being a couple, we almost always find ourselves with that extra seat taken. I have become so frustrated with this, that we now book window and isle seats on all our flights. Sometimes the seat between us is vacant, other times not. We usually offer to move for the person stuck in the middle.