Kourtney Kardashian and Travis Barker Are 'Madly in Love': 'Like High School Sweethearts' (Source)

Kourtney Kardashian and Travis Barker's romance is one for the books.

While the Poosh founder "was never opposed to marriage" before getting together with the Blink-182 drummer, a source tells PEOPLE, "her thoughts definitely changed when she and Travis became official. Those two spend all their time together and are madly in love."

The Keeping Up with the Kardashians alum, 42, and her rock star fiancé, 46, have been inseparable ever since they took their friendship to the next level earlier this year. PEOPLE confirmed the couple's relationship in January, and Barker and Kardashian went Instagram official shortly after spending Valentine's Day weekend together.

"She's over the moon," the source continues. "They're almost like high school sweethearts all over again. Neither of them ever thought they'd fall in love like this again."

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One of the reasons Kardashian is so smitten with her fiancé is because of the way he treats her children, Mason, 12, Penelope, 9, and Reign, 7, whom she shares with ex Scott Disick.

"He's so sweet and loving towards her and her children," the insider tells PEOPLE. "Their families also blended easily and nicely, and they all get along so well."

On Oct. 17, Barker got down on one knee and popped the question to his partner of almost a year in a romantic seaside proposal in Montecito, California.

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"Travis took Kourtney for a walk, and you could tell that she wasn't sure what was going on when she saw all the roses on the beach," a source told PEOPLE at the time. "Red and white roses were shaped into a heart. There were a lot of candles. They both looked very happy."

Added a family friend: "It was a complete surprise for Kourtney. Travis told her family that he planned to propose, but everyone was great at keeping it secret."