How do girls have the ability to shut you out completely?

I just came out of what I thought was a mutual breakup.. and now there is no contact, I have made an effort to be her friend – but she went as far as deleting me from facebook.

(nothing went wrong, just timing and circumstances were bad, we cared about each other a lot.. or so i thought) I just don’t get it…

is this some sort of defense mechanism to avoid pain and heartbreak?