Indeed.
For me, it is not the visceral, deep pain when someone you knew directly or cared for dies.
It is the cruel, sudden reminder that we are still all humans, with human troubles and human dangers to our lives. However much we may be intellectually aware that there are humans on the other end of the chat, behind that voice on the comms, in that ship we fly alongside, we often tend to regard all but the closest of online friends as, on some level, detached from the rest of reality. We are, after all, brought together primarily by a game about flying spaceships in an alternate galaxy.
I see this in some of the snippets of messages from other goons that have been posted: They talk about him repeatedly going AFK due to mortars or alerts all the time, then coming back later. Even though I'm sure they didn't doubt he was actually in mortal danger, they came to regard it as something that ultimately couldn't detach him from them for longer than a few hours.
And then in a second, those people are gone - not "yeah, taking a break for a bit to deal with RL", not "be right back, got to deal with something" - gone, gone, gone. And it's a nasty wake-up call for those who've not had this happen before, and even some who have.