In Real Life...
It used to be that everyone we knew was someone we had met in real life. At least that's what our parents, and most of our professors and bosses, tell us. Unlike the uphill-both-ways walks to school, though, these stories seem credible. And we certainly still do meet plenty of living, breathing people, and when we meet them we can even confirm the part about the breathing.
We meet people from all sorts of places. Some of them live in our residence hall, or apartment complex, or maybe just down the street. Others are in classes with us, or from work. Once in a while we might meet our best friend ever at a party, and if we have a great memory we will remember them within the next week. Some of them we even originally knew (gasp!) from the internet and have actually bothered to meet for real. The point is, anywhere we go, we can meet someone new.
When we meet people, though, we tend to make a little box in our head for them, remembering what they look like, maybe their favorite music or their sense of humor, and all of the other dirt we have dug up. Then we take this person's life story and put it in a pile with everyone else we met from the same place. One pile for roommates, a few for each class we're in, a small dust-bunny for people we've successfully dated, a landfill for people we've had less success with.