WhenJennifer Flavinfiled for divorcefromSylvester Stallonein a Palm Beach, Florida, courthouse Aug. 19, the move surprised many.

Just three months before, the pair celebrated their 25th wedding anniversary with gushing his-and-hersInstagram posts. And Stallone himself confirmed in July the family was filming an upcoming reality series for Paramount+.

But behind their smiles and happy statements, trouble lurked. Stallone, 76, and Flavin, 54, “are very different people, age-wise and in temperament,” according to a Hollywood source who has spent time with the couple. “What was once exciting and challenging is now routine and irritating.”

His apparent impetuousness could be a problem, a Flavin source tells PEOPLE. “He tends to do things on a whim without asking her before,” says the source. Indeed, Stallone admitted to TMZ he and Flavin had butted heads over his new Rottweiler, Dwight, butdenied the dog was the root causeof their split.

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Stallone rollout

Nevertheless, adds the Hollywood source, “disagreements on little things add up after so many years together.” That echoes what another sourcepreviously told PEOPLE: “It really wasn’t just one issue that made her file for divorce. They have had lots of issues for years and she just had enough.”

For more on Stallone and Flavin’s breakup, pick up this week’s issue ofPEOPLE, on newsstands Friday.

Stallone rollout

But Stallone is fighting Flavin’s claim. His legal teamrespondedto her divorce filing Aug. 29, stating in court documents that the actor “has not engaged in any conduct which constitutes intentional dissipation, depletion and/or waste of marital assets.” He is also contesting her sole use of the house.

source: people.com