Solution.

So. It's obviously been a little while since I posted last and yes, I have been living the whole time. (I always have to start off my posts with a disclaimer that I'm still alive- that can't be good.) But in the interim, I think I finally solved my personal quandary that I mentioned in my previous post. I thought and I thought, pondered and considered; and I now have a satisfactory (for me) answer.
It's nothing fancy...pretty straight-forward actually. And I like it because of that. Occam's Razor (of several possible solutions, the simplest one is preferred) holding, maybe this is the answer, although I won't say that outright.

The reason those people come in and out of our lives is to make us recognize and appreciate those around us who never fade away. Those persons who we think will remain a part of our lives forever and then...don't...serve as something like a background a basis for comparison. A control, in the scientific sense, maybe?
I've struggled as to how to explain this in the most clear fashion possible because I have this perfect mental image in my mind's eye. There's a black background made up of hundreds and hundreds of people, but they are only silhouettes. All around them is white. And standing up front, closer to me and in full color and detail, smiling, are those people who have ALWAYS remained present in my life- Mom, Dad, Kelli, my best friend from New Jersey- David, others, and a few new additions that I hope will stay there. And those silhouettes form the basis of comparison...or maybe the other way around...maybe those important people form the basis of comparison....but I don't think so. Those people that are smiling warmly at me have always been the exception; when others failed to be there for me in the way I imagined they would (the silhouettes), the important folk rose to the occasion and were there for me.
And sometimes people let you down. And then they fade to the background; it's not that you don't associate with them anymore, it's just that they're not on that level anymore. It sucks to have to "do" that, but that's why those who remain in full technicolor are THE most important people in the world to you.
I first realized this when my best friend, Dave, come in to town for my commissioning. And from the moment he first arrived at the airport, we were laughing and joking like we were kids again playing in my backyard. And not only did we laugh, we had serious conversations too...the war, the Marine Corps, etc. And I knew that people like this don't come along very often. And that's why they stand out.
So that's my explanation, and I'm happy with it. Maybe God's sitting back and smiling sympathetically at my creativity, but knows that I'm way off track :) It's ok though. I'll find out the real answer at some point.