Seal thinks of Heidi Klum as his "best friend" more than his "lover".

The 'Kiss from a Rose' hitmaker - who raises four children Leni, seven, Henry, six, Johan, four, and two-year-old Lou with the supermodel - believes a strong Friendship is the key to a lasting relationship because respectful partnership can be built on it.

He said: "I look at my wife as my best friend, the mother of our children second and my wife and lover third; because as wonderful as love is, it can get curly pretty quickly. But when you respect your best friend there is a line which you rarely, if ever, cross."

While the 48-year-old musician is blissfully happy with his wife, he is pleased they didn't meet until later in life as he doesn't think he would have been mature enough to make their relationship work when he was younger.

He added to the new issue of Cosmopolitan magazine: "I think most men are useless before the age of 40. If I'd met my wife Heidi in my 30s, it probably wouldn't have worked, due to my immaturity. I was 40 when we met - she was a fashion model and three weeks pregnant [by her ex-boyfriend Flavio Briatore] although she didn't yet know, and I was a singer in a profession not known for people being responsible. But when we fell in love we saw something in each other that transcended everything else."