I've been thinking about it, and over the years when coworkers have left, either through retirement, taking another job, or getting fired, I realize that outside of the office, they really had no significance in my life, and they just fade away.
If I think back on it long enough, I'll usually remember them, but again, it's not like their existence has any bearing on my life. One of my first wakeup calls was when the guy who had been my supervisor from 1993 to 2001, retired. He was a good ol' boy from West Virginia, retired Air Force, and tended to rub people the wrong way. But, I got along great with him. He had a trailer on a permanent campsite in Southern Maryland, and also had a boat, and always talked about having me come down some weekend. But, it never happened.
And, once he retired, I think I heard from him twice. Both times by email, and both times asking for information about something. Not a hey, how ya doing or anything like that. The first time, I got him the info he wanted. But then the second time, I just ignored it. And never heard from him again.
I sort of liken it to that old Twilight Zone episode, "The Bewitchin' Pool." One line that always sticks in my mind is "Those voices come in loud and clear for a bit. But, over time they fade away, and before you know it, you don't hear 'em at all!" Or, something like that.
That's pretty much how I've felt about work ever since then. Usually, whenever someone retires, or transfers, my first thought is usually "how incompetent is their replacement going to be?" but the person leaving, even if it's someone that I liked, is usually forgotten somewhat quickly.