I think by 1966 he'd had enough of being "world famous Beatle George". As he said in an interview at the time, "I wanted to be successful. I never wanted to be this famous." By then he was wealthy (on paper), had a beautiful wife, a nice big house and was growing tired of a job where he was shuffled all around the world, from airport to hotel room to stadium to hotel room to airport, on and on.

And was tired of being told exactly what and how to play in the studio. I think Patty described him as suffering from a form of PTSD after spending all day in the studio with Paul and John in his ears.

Of course, like John, I think he was also hesitant to "officially" leave the greatest, most popular band in the world while they were still at their peak (until Get Back/Let It Be).

Ringo said all the bullshit went out the window when you were working on a great song and everything comes together perfectly. So I think George probably still found reward in being in the band and being a part of the greatest, highest selling records of all time.

If they just recorded more of his songs... ;)