Awkward Encounters

Today a man said good morning to me as he walked past me on campus. A bit startled because I didn't recognize him, I said hello back.

And then I wondered, should I feel flattered because he said hello to me even though he didn't know me?

Or should I feel awkward because he said hello because he did know me but I just didn't recognize him?

Ah, the quandaries of my life...