Lives drifting apart, I guess. Or perhaps just mutual lack of sustained interest. Nothing too dramatic.
If you could be a writer for a living, what would you want to write about?
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It is easy to go down into Hell;
Night and day, the gates of dark Death stand wide;
But to climb back again, to retrace one's steps to the upper air –
There's the rub, the task.
– Virgil, The Aeneid (Book VI)