"It was honestly a classic case of marrying young, having a kid young and growing apart over the years," says source, who is close to Simpson. (The couple wed in May 2008 when Simpson was 23 and Wentz was 28.)
For the famous young pair, the demands of career versus family also caused strain on the relationship. "When you don't spend a whole lot of time together, things become challenged," says the source.
Since giving birth to son Bronx, 2, Simpson has kept a low profile to focus on family, but the singer-actress missed working.
"Ashlee was the one with Bronx a lot of the time, and that's a lot," the source tells PEOPLE. "Ashlee loves being a mom, but she also loves being a performer and she wants to badly get back to that."
Meanwhile, Wentz, 31, "was away from [Ashlee and Bronx] a ton working himself, and the balance was just off,"
Another source told PEOPLE Wednesday that things between the exes are "very amicable. "[Ashlee and Pete] are on very friendly terms and speaking – it's fine."