Kate Middleton, who turns 40 Sunday, would never have put herself in the spotlight — if romance hadn’t changed the course of her life, friends said.

Middleton — who will be queen consort when husband Prince William is crowned King of England — is planning a low-key celebration this weekend.

The quieter affair is more in line with Middleton’s preference, said friends who told the Daily Mail, “There is no way Catherine would be doing anything in public life if she hadn’t fallen in love with William. She is very open about that. Public life is full of extroverts, people who seek the limelight and enjoy it. And she isn’t one of them.”

Friends of Middleton’s, apparently authorized to speak to the Daily Mail, described William as the love of Kate’s life, adding that they are homebodies.

“Catherine and William are most in their element when they are out in their wellies in the sticks, muddy and scruffy with the kids,” the source said.

Prince William and Middleton met at St. Andrews University in Scotland in 2001 as freshmen. They both graduated in 2005 and posed for what became a famous picture of them sporting matching button-down white shirts.

“They met at the beginning of university,” another friend of Middleton’s said. “They were still young and have grown up together. And they still get on really well, they have similar interests and they enjoy doing the same things.”