I'd rather learn from one bird how to sing
than teach ten thousand stars how not to dance
i've always enjoyed.
and that line speaks to me of regretting the things you've done, rather than the things you haven't done.
although it has no mention of regret at all. maybe regret is such a human thing that it's a given somehow. we can see the opportunity costs of our decisions eventually and so the possibility of the bitter sweet hangs in the air