Somebody I used to know

Explain something to me. How do you manage to walk passed someone who once meant something to you, and have them look right through you? Blank stares. As if the relationship you had, meant nothing. How do they manage to put aside the fact that you knew them so well, that you opened up to them because they made you feel safe, and loved, and had lead you to believe that  you could trust them? Are they oblivious of the hurt they brought upon you? Or is it a pride thing? How do they bear being a stranger to someone they once cared for (or claimed they had)? Just something for you to think about... (because it boggles the crap out of my mind)