If I have a large group of people or am carrying large objects, I'll want a big car. If I'm by myself, I want a small car. The companies give you an “upgrade” when they are out of the car you reserved. It may or may not be what you prefer.

The airlines, on the other hand, now seem to assume that everyone “wants” an upgrade. In the past, they dealt with upgrade requests. I guess the new system is easier for them. I’ve gotten upgrades I didn’t ask for from four airlines, but mostly United now. When United adopted the UDU (now CPU) system, flyertalk had a poll. I don’t remember the exact vote, but there was a considerable fraction of people unhappy with the new system for a variety of reasons. Twice I went through the slight hassle of trying to remove myself from the upgrade list. It didn’t succeed. I won’t bother again. I certainly understand why a large or tall person would want the extra space. The world is really a better place if they get the larger seat and I don’t. But the other perks of upgrades (unpredictable meals, better magazine selection, someone hanging up a coat, great service (?), sitting in front of a curtain, bragging rights, ???, …); I don’t get why they are so desirable.

Some posters assume you can always trade your upgrade. One time my wife and I traded seats. She was challenged by the flight attendant. It made her feel just uncomfortable enough that she will never take it again.
I agree that not getting my window seat, or not sitting with family are not important problems in life. In every airline forum I checked, there are 250 threads with “upgrade” in the title, and most of the concerns there seem similarly trivial.

I note that I've been hit with a clever retort in this thread that I can only assume is some reference to a song or something from popular culture. I regret that I don't understand it.