Since Ashlee Simpson and Pete Wentz filed for divorce, no mistresses or cheating allegations have come forth. There have been rumors that Pete didn't want to call it quits, but Ashlee was reportedly fed up with her hubby's "erratic" behavior.

Now, a source close to Simpson tells PEOPLE that two simply married too young and weren't ready to balance their demanding work schedules with family life. "It was honestly a classic case of marrying young, having a kid young and growing apart over the years," says the source.

Ashlee and Pete married in May 2008, when Ashlee was 23 and Pete was 28, and their son Bronx Mowgli Wentz was born in November 2008. (Yes, do the math. Ash was certainly no virgin bride like big sis Jessica claimed to be when she first tied the knot!)

Ashlee has kept a low profile to focus on raising her 2-year-old son, but she's missed working and being in the spotlight. "Ashlee was the one with Bronx a lot of the time, and that's a lot," the source says. "Ashlee loves being a mom, but she also loves being a performer and she wants to badly get back to that."