Johnny McDaid felt "uncomfortable" with Courteney Cox's friendships with her male co-stars.
The couple are said to have secretly separated a month ago, almost 18 months after getting engaged, and insiders claim one of the factors behind their split was the Snow Patrol rocker's unease at the fact the 'Cougar Town' actress - who previously dated her co-star on the show, Brian Van Holt, and met ex-husband David Arquette on the set of 'Scream' - is very "close" to the men she has worked with.
A source said: "He's actually the one who instigated the breakup.
"Johnny feels she's a little too close for comfort with her male co-stars. He doesn't like the fact that she's close to them, that she stays in touch with them [because] she has this past of falling in love with people she works with. He feels very uncomfortable."
Additionally, the 51-year-old actress - who has an 11-year-old daughter, Coco, with David - is said to despise Johnny's smoking habit and the pair even underwent counselling to address the issue.
The source told the New York Post newspaper's Page Six column: "She has an aversion to [him smoking cigarettes]. She feels like it's a bad image, she doesn't like the smell of it. They constantly fight about it.
"He started vaping, she doesn't even want him vaping."
The former 'Friends' star has been left devastated by the split.
The source said: "Courteney is super upset about it. She wants to be with him.
"She's told everybody that this is the most she's ever been in love with anybody, more than with David or any of her exes."
It currently seems unlikely a reconciliation is on the cards as the former couple are "not on speaking terms" and thought it has been reported the 39-year-old musician has returned to Northern Ireland, Courteney "doesn't know" where he is.
The source said: "He upped and moved. She doesn't know where he lives.
"It's really bizarre how he's handled things. He's very cold, completely closed off from her."