I was reminded of this by a thread in the "That won't be possible" folder:
A couple of weeks ago, I was flying home from Wisconsin to Seattle, via O'Hare. I hadn't been able to get a seat assignment before the flight. When I checked in at the airport, I was assigned a middle seat, but near the front of the plane.
I boarded, saw that the window passenger was already on board, and settled in. A couple of minutes later, the person with the aisle seat got on. He asked if I would like the aisle. I blinked, accepted, and thanked him.
Yes, he likely had his own reasons, but it definitely made my afternoon easier, and was completely unexpected: who offers their aisle seat to a stranger with no obvious need for it?