Maybe it comes from having long legs and needing a place to put them. Or maybe from spending way too much time on my posterior. Whatever it is, I'm vowing to change.
The reason: I've come to realize something I knew all along but which my forgiving back has let me ignore: that young slouchers may look all limber and cozy, but old slouchers look pained.
I'm sitting up straighter, aided by a fine office chair that encourages good posture. When I'm not there I'm either standing (as I am now) or putting pillows behind my back to keep myself upright.
It will take a while, this shift in posture. But I'm ... slouching toward improvement.