^ Actually the more I think about the reasons put forth by everybody, the more I think it *must* be reality. And then I get more confused as to why Nolan would choose to end on the reality/dream question-mark. It makes it nothing more than an affectation rather than any genuine conundrum.
And therefore I go back to the there-must-be-more-reasons-for-it-to-be-a-dream way of thinking... But what? Except that he deliberately sets that up.