Very good summary.
My Dad, who wasn't allowed to go with his mates to WWII, never heard his Dad say anything about WWI. He'd never discuss it and the only time he even mentioned it was when my 15 year old Dad asked for his signature to sign up. The exact words escape me but it was along the lines of "After what I saw in the War, there's no way I'm signing you over to those Bastards!"
Years later, my English Grandfather came out to Oz, 1963 I think it was. When he met my paternal Grandfather, they quickly established which battles they were in, and they even realised their units were on the same battlefield during certain actions. My Dad sat to the sides listening to this banter, fascinated by stories he'd never heard before. As you'd imagine, the shutters went back up after he'd gone home.
It's clearly the shared experience that opens up the conversation. As a non Military man, I can only liken my experiences during the Black Saturday bushfires and, by no means do I intend to compare combat with fighting a bushfire*, but that's as 'exciting' as it gets for me. Our Driver saw some horrors on that day and he is still dealing with them. I was exposed to none of it as I was sheltered in the back of the truck but, he'll only open up to me "because you were there." He's told me things that I wish I never knew but I will listen as long as he needs someone to talk to.
It's the shared experience that allows him to open up & let some of his Demons out I guess. Best I can do is listen.
* Bushfires can kill you, but they lack malice, or the will to do so.