I was a UA loyalist until the 2012 merger and AA made a good offer to us to move over. It was great at first, but then a bit rockier recently. That said, it still currently works for me for my personal reasons (nonstops from LAX to where I fly, OW partners that fly where I want to fly) that may not necessarily be the same for others. I'm reluctant to go back to UA but have started dipping my toe in the DL waters.
I think it's a lot easier when you don't actually expect the airline to give a hoot about you and your business. So long as they deliver what they promise (get me there in one piece, reasonably on time, and in MCE with the occasional upgrade), I'll put up with the rest. It's a lot harder when you think they actually care, or should care.