John and Paul's mother issues
Mother issue had always been an important connection between John Lennon and Paul McCartney. Except for the experience of losing their mother at a young age, another thing John and Paul had in common is that they both admitted their sexual thoughts about their own mother.
In a tapped conversation, John recalled a moment when he and his mother lying on the bed at the age of 14: he had his hand on his mother's breast as he always thought he should do.
Coincidentally, Paul admitted that he was turned on by his mother when he was a teenage boy:
Paul also had other, vaguely sexual memories of his mother.
PAUL: At night there was one moment when she would pass our bedroom door in underwear, which was the only time I would ever see that, and I used to get sexually aroused. Just a funny little bit. I mean, it never went beyond that but I was quite proud of it, I thought, 'That's pretty good.' It's not everyone's mum that's got the power to arouse.
— Paul McCartney: Many Years From Now
That was not the only time they (subconsciously) confused their mothers with sexual partners. They both regarded their wives as mothers after they got married. John, who called his second wife Yoko "mother", undoubtedly had this tendency. Paul also had a strong dependence on his wife Linda, his marriage was regarded by many as a mother-child relationship:
[Paul’s] a mummy’s boy who didn’t have a mummy after his mother died when he was 14. He would be lost without Linda now.
— Denny Laine
Linda was the only one Paul could open up to, she was like a mother to him. I have no doubt she loved him enormously.
— Peter Cox
Apart from the innate dependence, their attachments towards their "new mothers" deepened by the growing problem between two of them. The mother figures - Linda and Yoko both showed up at the period that John and Paul were drawing apart. They naturally turned to their "mothers" for comfort and strength after being hurt by each other, which, unfortunately, worsened their relationship cuz they tended to blame the other's girl for taking their partner away.
