We used to be friends

There's a comunity on orkut that says "I Search for for [fake] keys in my purse to avoid acquainteces".
We've all been there, You're walking at the mall, when you see someone you "sord of know", and first you "panic" and doesn't really know what to do, and then you wonder:
"Did she see me?" "Should I say Hi?" and after considereing all the pros and cons you just start "searching" for something in your purse, or turns the other way and acts like you just didn't see the person. [That last one really works if the person you are turning away from was also whising not so greet you.]

The cenario is almost always the same, what changes is the people who are part of it.
Maybe you and you're acquaintece were friends at 8th grade and for some silly reason got into a fight and never spoke to eachother again. [and not only you feel horible for not saying "hi" you also get stuck thinking "why you two aren't friends anymore", and "what would be different if you still were. But after a few moments you forget all about it.

Maybe they're just a friend of a friend [and you don't really know if they remember you]

Or maybe you two were friends not that long ago, and for reasons you both know well.
After you parted ways with that person you thought "hey, maybe I'll never see him again, and this won't be so hard" [which you knew all along was just whishful thinking]. And when you least expect, you're looking at his face, with no "preparation", no speech planned out, nothing.
Just a million thoughts running through your head, and all sorts of fellings, you might feel a little mad at first, just to cover up the sadness you know its there.

What I'd love to know is when the last cenario becomes the first one, when do the feelings turn into just memories, when does the sadness goes away.