(total) recall

I find that increasingly often I'll pass someone by on my way to work or class, recognize his face, but not be able to remember where I might have seen him before. It's odd to do that double-take and to feel some sort of bond with a person who may as well be a stranger (that is, if my reluctant memory has any say in the matter). I wonder why I can't muster the ability to recall a name to go along with that face, particularly if that face is significant enough - for whatever reason - for me to retain and retrieve some kind of emotional association for that "anonymous" person. Where is the filter? Why does certain information stick, while other bits fall away indefinitely?