While Stiller, 51, and Taylor, 45, have collaborated over the years, the actor’s hectic work schedule put a strain on the relationship.
“It gets hard — he has a very busy Hollywood career, and she’s been very focused on being a mom,” a source tells PEOPLE in this week’s issue.
“They are a very loving family,” adds the source, who doesn’t expect a messy uncoupling. “[Taylor] and Ben respect each other and will do anything to make sure their kids are happy.”
For more on Stiller and Taylor’s decision to end their marriage, pick up the new issue of PEOPLE, on newsstands Friday
The former couple announced their decision to separate on May 26 in a joint statement to PEOPLE.
“With tremendous love and respect for each other, and the 18 years we spent together as a couple, we have made the decision to separate. Our priority will continue to be raising our children as devoted parents and the closest of friends. We kindly ask that the media respect our privacy at this time.”