"It can be quite hard to say 'I love you' to someone," he says. "I felt this with Nancy but to say it wasn't that easy."
So the former Beatles star decided to tell his wife Nancy Shevell – whom he wed in 2011 in a low-key ceremony at London's Marylebone Town Hall attended by just 30 guests – those three special words the way he knows best.
In his latest album, New, is a secret piano ballad called Scared, which, Paul says, is for Nancy.
"There are a couple of tracks about Nancy and Scared is one of them," the 71-year-old tells The Sun.
The beautiful ballad, which comes from the heart, opens with the touching yet simple line, I'm scared to say I love you.
The music legend's 16th solo album is fresh, youthful, and, as its title reflects, shows a new wave of energy from the music legend, who lost his wife Linda to breast cancer 15 years ago and more recently divorced Heather Mills.
It was produced by young talents Mark Ronson , Paul Epworth – who has worked with Adele – Ethan Johns (Laura Marling, Kings of Leon) and the son of George Martin, Giles.
Many tunes are a nod to Sir Paul's past, in a move to own his 'personal history', which, he says, people unwittingly steal.
On My Way To Work remembers Paul before he was famous, on his way to work at "an unearthly hour".
"It’s about me going to my first job, before The Beatles took off, which was working on a lorry for a delivery company called Speedy Prompt Deliveries…SPD."
Paul explains what he's trying to say with his latest album, set to be released next week: "I'm trying to say, 'Hey listen guys, I was there. It was me sitting in that room. It was me walking along the street.'"