The Beatles star said he and Lennon “grew up together” during an on-stage interview at the Southbank Centre in London.
When asked about his love for Lennon, Sir Paul said: “As 16-year-old, 17-year-old Liverpool kids, you could never say that. It just wasn’t done.
“So I never did… really just say, ‘John, love you man’. I never got round to it. So now it’s great just to realise how much I love this man.”
Listen to ITV News's Entertainment podcast Unscripted
He added that spending his youth with Lennon “was like walking up a staircase and we both went side by side up that staircase”.
“I just remember how great it was to work with him and how great he was,” he said.
“Because you are not messing around here, you are not just singing with Joe Bloggs. You are singing with John Lennon.”
Sir Paul was also asked about the response to the news Liverpool has been handed £2 million in government funding for a new Beatles attraction.
“I know the people from Japan and America and South America all love the Beatles,” he said.
“So if they come to Liverpool, that’s a lot of what they are coming to see, you know. I think it’s fine.”
He added: “I’m quite happy that they are recognising that it’s a tourist attraction.
“But I think the could also spend the money on something else.”
Sir Paul made the comments during an interview with journalist Samira Ahmed to promote his new book The Lyrics.