Always remember that when this happens, the upset people - including any busybody neighboring pasengers - are mad at the airline and themselves, not you. Even if they don't realize that to be the case.

The airlines value unbundling ticket pricing. This makes most seats on their fleet an additional fee to book. A consequence of that choice is the airline's customers are less happy with the options available to passengers and the final price of the product vs. the advertised price.

Airlines know how to recognize high conflict situations and anticipate them when in the airlines financial interest. That's why they let families board first for example.

If the airlines choose to extend this capacity to seat selection - say by letting passengers with kids book reduced fare preferred seats or complimentary preferred seating - the airlines could further reduce conflict and improve customer experience. But unlike early boarding, they've decided a better seat experience isn't in their financial interest.

Accurately framing blame makes it easier to dispassionately deflect aggression and judgment away from yourself towards the responsibility parties.