How do people just get up and leave?

You have a handful of memories, over months, over years, let's say. Even if someone were to push you off the roof, and make you fall in this pit where darkness wasn't the only uncomfortable thing around, how DO you just get up and leave?

I fell into those kind of pits millions of times. But. I could never be convinced enough to close the doors and wallow in the loneliness that I would be put through. I always thought nobody had the balls to presume that it was all over. Another shot at the trust, perhaps? Maybe, just MAYBE you would have wanted to give the person a chance to justify?

Part 2: Let's suppose you fall into the pit a million times, give the person chances, realize that in that relationship, the pit was your second home and finally close doors.
How much does it hurt to open the door, every once in a while for a peek, to see the other person walked away the very moment you made a stance?
I know, I should have seen it coming. But.

How do people just walk away like that?

Note to self- I must buy myself a heart of stone. Apparently it's the new trend.