John Lennon had deep-seated mommy issues. His mother Julia was a "free spirit" party girl who left young John with her stricter, more conventional sister Mimi and her husband (John's father was a merchant seaman and was not a part of his life). Julia remained out of John's life until he was a teenager. Julia encouraged his art and his guitar playing, which Auntie Mimi discouraged, and even taught her son banjo chords (that are very different than guitar chords and which Paul McCartney would have to break John of later).
John loved having his mum back in his life. Sadly, she was killed by an off-duty, drink-driving cop when John was around 16. He suffered because of that tragedy throughout his young-adult life. He was a very angry young man, leading to risky, erratic, and violent behaviours.
These violent tendencies often manifested against the women in his life. John was a batterer, both of his first wife Cynthia and later of Yoko. Cynthia was a typical Merseyside girl that John only married because he knocked her up with their only child Julian. John wanted to be the father to Jules that John never had, but Julian was born just as Beatlemania was taking off.
John met Yoko via an art show in London in 1966, and was instantly smitten. Yoko, who was several years older than John and a feminist, became for him a mommy figure. But having her in his life didn't resolve the deep issues in his psyche. All of the drugs that were a part of that scene didn't help. He couldn't ever talk about his pain of his childhood because that wasn't the English way. When the Beatles were in India, one of the songs John wrote (with a finger-picking style taught to him by Donovan in India) was a love song to Yoko but titled with his mother's name.
After the Beatles broke up (not BECAUSE of Yoko, but she played a role in the clash of personalities and egos that caused the split), John and Yoko, now married, underwent the now-discredited Primal Scream therapy of psychotherapist Arthur Janov. Although the therapy didn't "cure" him, it did leave us his 1970 masterpiece album "John Lennon/Plastic Ono Band".
In the early 70s, the Lennons, now living in NYC, got deeply political, spurring J. Edgar Hoover's FBI to start surveilling him. This would lead to the Nixon Administration to start deportation proceedings against John, made John increasingly paranoid. This did not help his emotional issues.
His physical abuse against Yoko and overall assholish behaviour (in small part because of a bruised ego of being the only ex-Beatle at the time not to have a No. 1 hit single) prompted Yoko to have him out. This led to his 18-month "lost weekend" in LA in 1973-74, getting blitzed with the likes of Harry Nilsson, Ringo, and Keith Moon. John constantly called Yoko begging to come home, but she refused. This was Lennon's rock bottom.
During this time, his friend Elton John, the top pop act of the time, played on and contributed vocals to John's song "Whatever Gets You Thru the Night" for Lennon's 1974 album "Walls and Bridges". Elton assured Lennon that the song would reach No. 1. Lennon didn't think it would, so Elton made a bet with him that if the song topped the singles chart, John would have to appear on stage with Elton. To Lennon's surprise, the song did go to No. 1, and the date was set for John to play a three-song encore at Elton's Madison Square Garden concert on Thanksgiving 1974. Unbeknownst to Lennon, Elton arranged for Yoko to be at the show. Sickened by drink and stage fright, Lennon made it through the short set. After the show was over, John saw Yoko looking at him backstage. It was then that John and Yoko began making arrangements for him to go back home.
Shortly thereafter, Yoko became pregnant for the second time with John (the first pregnancy ended in miscarriage). During this time, Lennon's immigration status was reserved, with a U.S. Court of Appeals ruling that immigration officials and the FBI acted improperly with regard the Lennon's case. On October 9, 1975, Lennon's 35th birthday, John got his green card and Yoko delivered John's second son Sean.
John would go on to drop out of show business for the next five years, being a stay-at-home dad and making homemade recordings while Yoko ran the family business. Being a homebody and NOT having to be a rock star was just what John needed. He was finally happy, and, more importantly, he was rid of his demons.