McCartney expanded on what haunted the musician who tragically died in 1980.
"He had [a paranoia] that people died when he was around: His father left home when John was 3, the uncle he lived with died later, then his mother died. I think John's whole life was a cry for help," he added.
McCartney, 73, gave the interview to talk about and commemorate some of the band's No. 1 hits over the years.
On how the song "Help" came together, McCartney said, "We finished it quite quickly; we went downstairs and sang it to John's wife at the time, Cynthia, and a journalist he was friendly with called Maureen Cleave. We were very pleased with ourselves."
Another song that even the most casual Beatles fan knows is "Hey Jude" in 1968. It's famously inspired by Lennon's son Julian.
"I was on the way to see him after John and Cynthia got divorced, and because I was good friends with [Julian], it came into my mind: 'Hey, Jules, don't make it bad,' " he added. "It's a song of hopefulness."
Obviously Jules was changed to Jude and the rest is history.