A young man with a family at home, he was considering his options - he’d been a Labour Party member since his teen years - as he wound down his army career.
When his wife Caroline was diagnosed with bowel cancer, he found the decision forced upon him faster than he’d prepared for.
In 2010, ten years after they’d met - and a year before he would be elected as MP for Barnsley Central - Caroline died of bowel cancer.
Dan attempted to use the coping methods he was taught in the Parachute Regiment, but grief was an altogether different type of stress he couldn’t avoid in the same way as bullets flying over his head.
“I found ways of changing the subject,” Dan told the Chronicle. “I didn’t feel ready to talk about it.
“I decided it was time to stop deflecting away those issues, and think about them rather than putting them away in this ‘coping box’.
“That is something I was taught to cope with a huge amount of stress. You can get through these situations by packing all your worries and concerns away.
“It’s something you find in men of a certain age and especially Northern men.
“Quite a lot of my friends from the armed forces have committed suicide. It’s deeply upsetting to think people were so consumed by depression that they didn’t feel they had another way out.”
The book, written over a period of five years in the ‘margins’ of his joint roles as MP and Sheffield City Region Mayor, depicts Dan’s journey from his first day in the armed forces, to his arrival in parliament as an MP.
In that period, the 47-year-old - who joined the Parachute Regiment in 1997 - served on operations in Kosovo, Iraq, Afghanistan and Northern Ireland.
He said: “The book is my story of how I worked my way through the process of grief.
“It wasn’t planned at all, but as I’ve walked about door-knocking and meeting people over the years I found myself having a lot of conversations about grief and bereavement.
“It’s quite a tough read and was incredibly difficult to write, but I was determined to get to the end.”
Dan, who remarried in 2013, said he found writing the book ‘incredibly rewarding’.
“To lose a loved one is obviously an extraordinarily difficult experience. I lost my wife at a very young age.
“I look back on it now, much older and wiser, and realise I’ve learnt a huge amount.
“What I try to do in the book is convey that it’s possible to work your way through and live what might be considered a normal life. I wanted it to be helpful to others in the same situation.
“For the first time I’d been able to try to make something out of the experiences. I did it with a degree of nervousness, because they’re very personal things.”
He said initial responses to the book, which is released on March 5, had been ‘very heartening’.
“A lot of people have come forward and said they’re struggling.
“Hopefully I’ve provided a bit of a roadmap on how you can find a way out of the fog of grief.”