From his interview with Kung Fu magazine:
"I’m not joking around when I’ve said occasionally, trying to learn how to play a D chord properly has been a very big thing for me. The exact way to get this tone, which I can now do all the time, has taken forever, it seems like forever."
Advanced Artitst are relentless in their pursuit of perfection. That's why Steve Miller takes months to record a three-chord song, and why it takes Lou Reed years to learn how to play one chord. Some people think it's silly, but you can't argue with the results.