Adam Shankman, one of the producers of 'The Last Song,' is dishing on how Miley Cyrus and fiancee Liam Hemsworth first met on set -- and why they're such a great match.

"I sort of want a finder's fee for Miley Cyrus and Liam Hemsworth," Shankman told PEOPLE. He explained that the couple's chemistry was immediate and apparent, despite Hemsworth having a girlfriend when he and Cyrus first met. "I remember that first day of shooting when they first kissed and Miley walked by my sister [producer Jennifer Gibgot] after they shot the kissing scene," Shankman said. "She goes, 'This is gonna work.'"

Why are Miley and Liam so perfect together? "I'll tell you what's so great," Shankman said. "That thing about opposites attract. Their fundamental core values are the same, but he's really mellow and she's really hyper. So it really works together." Indeed, Liam does seem to balance her out, especially when she gets caught partying a little too hard a little too often. And let's be real: If a man sticks around after you order him a penis cake for his birthday, well, he must love you!

But does Shankman think the mucho-young couple (Miley's 19, Liam's 22) are a little too short in the tooth to head down the aisle -- some sources are saying as soon as next summer? He wouldn't say, but he did tell the mag one thing that did freak him out a bit. "It's extraordinary, and I'm so happy," he said, "[but] I'm scared at how beautiful those children are going to be."