Therefore, if your friend insists that we must have something to mourn in order to create and appreciate poetry, consider the unavoidable slow loss of our past selves.

This happens to me now. I have more than once noticed and mourned gaps in my memory, where a day of bliss interrupted by some silly bit of emotional drama is retained only as a memory of a ruined day.

And on a more professional level, I routinely notice the annoying difficulty of recalling how a "past self" as little as one day old would think about a problem (when I've effectively deleted that self due to a recent mind hack).

(It's not that I personally care about how the deleted self thought or felt, it's just that I'm usually trying to write accounts of the before-and-after of my mindhacks, and it's bloody difficult to write the "before" part, after, because I just don't think the same way any more.)