Every once in a while I work on a project that doesn't use preprocessors. No variables, mixins, functions, nesting. Just pure CSS.

It is a challenge, at least for me. The most common mistake I make the very first second I open CSS file is I try to nest selectors. This feature is so simple and powerful that I don't notice it. It is part of my daily routine and getting back to writing old-school selectors is hard suddenly. It is hard to debug and search "flat" selectors, too. Maybe it is just me, but I cannot live without preprocessors anymore.

New developers are emerging every day. Do you learn preprocessors from the start or do you learn pure CSS first?
What about experienced developers, how do you behave when you write pure CSS nowadays? Do you enjoy writing pure CSS?
Did we take preprocessors for granted?