Why did they breakup?
OK, so most of you have probably heard the theories. Here are my thoughts:
Business difference
IMO, it can't be stressed enough that while Brian was alive, they had nothing else to do but focus on music and being Beatles. The business stuff created conflict and for the first time, they had other things to focus on other than music. They took it out on each other. Simple as that. You always take it out on those closest to you and there probably have never been a band as close as the Beatles.
Yoko
Yoko is the most common scapegoat because it's much easier to just blame a single person, especially someone like Yoko who is kind of strange and very annoying. The truth is, John himself said he started thinking about life without the Beatles as early as 1966, before he even met Yoko. Yoko's presence in the studio certainly created conflict, but John had to know it would since nobody participated in the Beatles sessions. John Lennon himself decided to change the unwritten agreement that they had lived under since 1963. With or without Yoko, I believe they would have broken up. Once John decided it was over, it was over. Yoko was just a symptom to another real problem, namely...
Life intrudes
Since 1960, the Beatles practically lived with each other. The four individuals within the Beatles had forsaken their own ego to the idea of the 4 headed monster, namely, the Beatles. Lots of pressure involved with being a Beatle and they finally got sick of it. I don't know about you guys, but when I was in college, I had a group of like 4 guys and we did EVERYTHING together. What happened?? Well, people start to lead their own lives. They get their own jobs. They get their own wives. They get their own families. Those friends are still important to you, but they take a backseat. That's what happened to the Beatles, except in a far more public setting. They wanted to lead their own lives. So, couldn't they just have treated it like a job?? See, that's the thing. The Beatles was never a job. It was four extremely close friends who made magic together.
Artistic differences
This is another big one. George Harrison, in particular, had grown to the point where he couldn't be contained in the context of the Beatles as they had existed. There would have had to be major changes in order for it to continue. The fact is, that George had always been looked down upon, a bit, by John and Paul. Not on purpose. They loved him, but if you've ever had a friend who was younger than you, sometimes you have a hard time thinking of that person as a grownup. Especially in the case of George, who in the early days, really didn't have the song writing skill to match John and Paul. By the time he developed those skills, it was difficult for John and Paul to think of George as an equal, or at the very least, not as an inferior. In addition, Paul and John's artistic differences had always been there. In the early days, those differences are what made the Beatles great. In the later years, the business differences and personal differences made it to where they would attack each other's music. So, John would say Paul wrote granny music and Paul would say that John's music was too harsh and not focused enough. Ringo was kind of caught in the middle of this, three extremely strong musical personalities and trying to keep them all happy.
The truly sad thing is that I believe that, eventually, they would have reunited. There is enough evidence to suggest that, despite what John said in interviews, he DID enjoy a lot of Paul's solo work and wanted to work with Paul again. I believe that they would have gotten back together. John was in a much better place by 1980 and his songs from that period even SOUND like Beatles songs. Paul and George also, I believe, came to grips with the Beatles sometime in the 80's. The '95 reunion, I believe, would have happened if John had been alive and with John alive, Lord knows what we would have gotten.