lengths of relationships

Maybe a silly question, but can someone answer why some relationships last longer than others ? I left a marriage of 17 years because I got sucked in by a narcissist. Over the top fun, pretty, etc. Ironically, she always told me, " she never wanted to date a married man."
Truthfully it was the best time in the whole relationship. Once my separation agreement came through, things started to change. She went from idolizing me to finding faults in almost everything I did. I almost changed every aspect of my life to accommodate her and nothing was ever enough. I was devalued and discarded, but would still take her back in a minute. I miss her terribly. It's very sad. I hear of these relationships that last for a long time. I don't think my girl would have ever gone through with marriage, ever though that was her plan at one time. Are some narcissist content as long as you put up with there crap ? Mine didn't put up with anything. Are there varying degrees ??