I was saying to someone the other day that one of the very first gigs we did – I don’t even think we were the Beatles, it was the Quarrymen – one the very first times I ever played with John, we did a very early gig at a thing called a Co-Op Hall, and I had a lead solo in one of the songs and I totally froze when my moment came. I really played the crappiest solo ever. I said, “That’s it. I’m never going to play lead guitar again.” It was just too nerve-wracking onstage. So for years, I just became rhythm guitar and bass player and played a bit of piano, do a bit of this, that and the other. But nowadays, I play lead guitar, and that’s the thing that draws me forward. I enjoy it. So, yeah, that means the answer to “Are you going to retire?” is “When I feel like it.” But that’s not today.
FROM https://www.rollingstone.com/music/music-news/paul-mccartney-the-long-and-winding-qa-242714/

At the deepest level, McCartney has little idea where all the melodies come from. He still hasn’t figured out how he wrote “Yesterday” in his sleep. “I don’t like to use the word ‘magic,’ unless you spell it with a ‘k’ on the end, because it sounds a bit corny. But when your biggest song – which 3,000 people and counting have recorded – was something that you dreamt, it’s very hard to resist the thought that there’s something otherworldly there.”
FROM http://www.rollingstone.com/music/news/paul-mccartney-yesterday-today-20120618