Well, since you asked....
I'll go a little bit into why I think George left, just my personal opinion, a lot of this is based on listening to the Nagra tapes, a lot of this stuff is not in the doc so hard to deduce otherwise. First, the very first day, I don't know if the cameras even capture this, John breaks out a little bit of "Everybody Had a Hard Year" (which would be incorporated into "I've Got a Feeling" later, but for a while it was its own standalone song...). Right after that, George give a couple of run-throughs of "Lay It Down", one of his most brilliant songs ever IMO. John doesn't react at all. No comment, no "hey, that's nice", nothing. John really was like a big brother to George, so I think that hurt his feelings.
Second, the one George song they did attempt, "All Things Must Pass", got a fair amount of work in, but they abandoned it after January 8. two days before George quit (same for "I Me Mine", it was only worked on for one day, January 8, the album version came from, and on a side note, I don't know why they abandoned that flamenco section in the middle, I love it, would have loved to have heard that in the final version...). So he's already starting to feel short-changed. In the meantime they spend tons of time working on "Maxwell" and "Bathroom Window" and "Long and Winding Road" and "Two of Us", all Paul songs.
Third, the whole, live, no overdubs concept I think really grated on him. He had "Let It Down", "All Things Must Pass", "Something" was coming along, all brilliant songs, and I think he knew they deserved better than this rag-tag approach that was going on. He was coming into his own and I don't think he wanted to "throw these songs away", he knew they could be really made special, strings, moog, whatever. So he was being held back in that regard.
Fourth, and the straw that broke the camel's back, was just Paul's bossiness. And not so much bossiness, but his insistence, ever how slight, to put his thumbprint on all their songs. He did the same thing to John, I can't remember which song, probably "Don't Let Me Down", where he tells John, "Let's try it this way" and you can see John is not having it, he's like "this is my fucking song, I'll do it the way I want". George had already been through this on "Hey Jude", ("I'll play if you want me to, or I won't play if you want me to, whatever it is to please you" is basically the quote). They had been working on "Get Back" and "Two of Us" all morning, with Paul basically directing how he wanted the song to go. I think George finally felt, fuck, do it yourself then. I can have a better time sitting in my garden writing songs and then fucking Patti at night. Although that actually may have had something to do with it as well, relating to George's overall state of mind, as Patti had left him at the beginning of the year.
and finally, like Ringo, I don't think he was into the project at all, you can tell he hated the cameras and microphones being around, "Oh so you're taping our conversations" he says right at the beginning. And just the entire trying to run their own business affairs was a drag, it all piled up on him. Just my $.02 anyway