The Beatles legend tied the knot with Shevell back in May 2011, with the pair beginning their relationship in 2007 as McCartney was finalising his acrimonious divorce from second wife Heather Mills.
McCartney, 83, says he and Shevell, 66, are so happy together because they are "nothing like each other" and that works well for their relationship.
Speaking on the Song Exploder podcast, he said: "We've known each other quite a long time and it's a very interesting relationship. We're nothing like each other. I'm English, she's American. She's very practical, gets things done, I'm much more sort of whimsical. I will get things done but maybe not in as practical a way.
"We know each other and we know how to be with each other."
McCartney has dedicated the song Ripples in a Pond to Shevell from his new number one album The Boys of Dungeon Lane.
He was thinking about how "blessed" he is to have Shevell as his wife when the lyrics and chords began to flow.
McCartney - who lost his first wife Linda to breast cancer in 1998, when the photographer was 56 - said: "I was thinking about my missus Nancy and thinking how lucky I am to know and love someone like her.
"I was just thinking about how blessed I am. Anyone who is in a good relationship with someone is inevitably really blessed and it's nice when you're thinking that to introduce that idea into a song."
If Shevell is the perfect romantic partner for McCartney, he also revealed his greatest ever musical partner was his late Beatles bandmate John Lennon.
The Get Back rocker was just 15 when he met a 16-year-old Lennon at a summer garden fête at St. Peter's Church in Woolton, Liverpool, where Lennon's skiffle band The Quarrymen were playing.
The pair were introduced by mutual friend, Ivan Vaughan, and after McCartney showed Lennon how to tune a guitar and play Eddie Cochran's Twenty Flight Rock a friendship that would change the course of music history was formed.
And McCartney admits he has never felt as comfortable with anyone as he did with Lennon - who was shot dead in New York in December 1980 aged 40.
Speaking on the Song Exploder podcast, he said: "Working with John Lennon was something that happened when we were kids. Neither of us really knew how to write songs so it just grew organically. Because John and I had such a natural relationship that had matured together I'm inevitably comparing whoever the next person is that I'm writing with.
"As time went on and The Beatles broke up and I was no longer writing with John I did try writing with a couple of other people. Having worked with John, now for someone to just come into the room we don't necessarily know much about each other it does make it a bit harder, I must admit."
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