Something I've said before that I've always found useful is to borrow a pair of terms from causation and to think about purpose in two ways: proximate and ultimate. An ultimate purpose would be just that; it would be an end point, a final terminator of all those "But why ...?" questions. I agree that a God might well provide such an ultimate purpose for humans ... but as you've just pointed out, in having no higher purpose a god itself would have no ultimate purpose.
In any case, I see no reason to posit such an entity so better - and realistic, and true, as far as I'm concerned - not to think of ultimate purpose but of proximate purpose
s (because there can be more than one purpose in life, can't there? Why wouldn't there be? It could even chop and change throughout the course of an individual's life - why not?). The things we do that give our lives meaning are ours (no matter with however many others we may share them) and usually - not always, but usually - end with our own lives. That's as it should be, to me; if you can't find meaning and purpose in the living of your life, you sure as shit ain't going to get it anywhere else.
The well-worn phrase "The meaning of life" ought to be erased from the English language AFAIC - the very phrase itself, with its definite article, implies that there's only one such meaning (
the meaning of life) which you can either succeed in uncovering, or not. This to me is nonsense on stilts. There are are more meanings and purposes in life than can be counted, probably. Human existence has no
ultimate meaning or purpose as far as I can see, nothing past (or as it's usually phrased, 'above' or 'beyond', whatever that might mean) itself to give it significance; individual lives may very well have, though. The people who become very good at making the purpose of their lives obvious to others tend to become famous; they become the great novelists and poets, the composers and the painters, the actors, the humanitarians and philanthropists and what have you.
A lot of this same ground was covered in an older thread, by the way:
http://goo.gl/VYsqJ9