Jan. 10, 2005 -- -- The breakup of one of Hollywood's most celebrated couples has entertainment writers around the world scrambling to answer the question why.
Peter Castro, executive editor of "People" magazine -- the publication that broke the story of Brad Pitt and Jennifer Aniston's separation -- told ABC News' "Good Morning America" that the decision was ultimately up to Aniston.
"A source close to the couple told 'People' magazine that it was Jennifer's decision to break it off," Castro said. "He [Pitt] was half a step behind her on this. He wasn't terribly happy either."
Castro says Pitt's desire for children likely played a large role in the couple's decision to separate.
While Pitt told ABC News' Diane Sawyer that he wanted children in a recent interview, Aniston wasn't singing the same tune, according to Castro.
"You could tell the emotion and see it in his [Pitt's] heart, this is what he wanted more than anything," Castro said. "But she wasn't there with him. The timing was off."
Pitt's publicist told "People" about the couple's plans to "formally separate" days after Pitt and Aniston jetted off for what seemed like a romantic getaway.
The couple joined Aniston's former "Friends" co-star Courteney Cox and her husband, actor David Arquette, on the Caribbean island of Anguilla over New Year's weekend.
Castro says it's likely that the couple decided to take the trip despite their plans to split.
"They knew long before they took this vacation that they were not going to remain together, or at least separate," Castro said. "And they decided, you know, we told Courteney Cox and David Arquette, their best friends, we were going on this trip ... let's go ahead and make the best of it and see what happens," he said.
Castro says "People" is releasing a special issue with the cover story "Why They Split" this week because the separation is such big news in Hollywood.
Since their lavish July 29, 2000, wedding, Pitt, 41, and Aniston, 35, have become the most powerful couple in the Hollywood film industry.