Most of the time, it's simply self-preservation. But there comes a time when we have the one person in our lives that need to decide if we're going to turn our backs on them or not. When it comes to our own livelihood and survival, the path of least resistance would lead us to turn our backs. But you need to be sure you have all the information, all the back story before you ultimately burn someone in your life and sometimes, someone you actually care about.
So choose who you turn your back on wisely: you really never know whose struggle you're making worse - or better.