Don't you think?

I was wondering the origin of the idiom, "Don't you think?" I know that it SHOULD be in the context of, "what were you thinking?", but we usually use it to mean, "do you agree?". Any comments would be nice, and much appreciated.

I've always assumed it was just an abbreviation of the much older "Don't you think so?" (VSD)

[It's easy to think of English colloquial usages, most in the form of questions, where some word meaning "the same" is unspoken yet understood. "We had good luck today, wouldn't you say (so)?" "I'm exhausted, aren't you (also)?" "Jerry and Terry are going, why don't we (go too)?" Et cetera. - Baceseras.]