Speaking to Lennon’s son Sean for a new BBC Radio 2 documentary to mark what would have been the late singer’s 80th birthday, McCartney recalled how the pair fell out when the Beatles split in 1970.
Despite their feud, they are believed to have reconciled before John was shot dead by Mark Chapman in New York in December 1980.
McCartney told Sean: “I always say to people, one of the great things for me was that after all The Beatles rubbish and all the arguing and the business, you know, business differences really… that even after all of that, I’m so happy that I got it back together with your dad.
“It really, really would have been a heartache to me if we hadn’t have reunited. It was so lovely too that we did, and it really gives me sort of strength to know that.”
McCartney also opened up on how the pair established one of the most acclaimed songwriting partnerships in history, but admitted that it wasn’t as successful in the band’s early days.
“There were a few songs that weren’t very good,” McCartney said. “There were a few that were clearly [by] young songwriters who don’t quite know how to do it. There was one called ‘Just Fun’.”
He added that he still thinks of Lennon while writing, despite his death 40 years ago.
“I’m writing something and I go: ‘Oh god, this is bloody awful.’ And I think: ‘What would John say?'” he said. “And you go: ‘Yeah, you’re right. It’s bloody awful. You’ve got to change it.’
“And so I’ll change it, and I know from reports that he did similar things to that. If I’d have a record out, he’d go: ‘Bloody hell… got to go in the studio. Got to try and do better than Paul.'”
Meanwhile, Lennon’s glasses and a detention sheet from when the late Beatle was in school have gone up for auction, marking 50 years since their split.