djayr,
I, personallly, had enough when I watched my alcoholic husband go back to drinking (with no apparent desire or inclination to stop) after I had nursed him through one alcohol-related near-death experience.
I was at a place of peace with my decision. I felt I had done what I could to help him have an opportunity to recover, and he chose not to take advantage of it. I wasn't willing to put myself through it a second time. He knew what to do, it was his choice whether to do something to stop or to continue his downward slide. I was sad for what might have been, but convinced that staying would not help him and might do me in.