...when it's well below freezing out and you've gotten used to the gentle breeze of
cold air from the defrost vents, because you want to keep as much heat as possible in the engine. (I'm intrigued by the silica sock idea...)
...when your car was never as free of clutter as it is now, considering that keeping it clean wasn't as much of a priority for parents or dates before.
Red Devil beat me to this thought, but I wanted to post it anyway:
...when you park facing south* in the hopes that the sun will keep your engine compartment a little warmer.
* north in the Southern Hemisphere, I suppose.