When do you draw the line?

Over the weekend I was talking (complaining) about a person who is always so rude to me. I've over compensated trying to be a friend to this person. There just hasn't been any reason to like them yet I react to them by trying to get them to like me. It's hard for me to understand why this person reacts to me this way I have lots of good friends and they like my flawed, opinionated self. My friend that I was complaining to said something that really got me to thinking, he said, that this person if very defensive about everything they do. And that really hit me, yes they are defensive and insecure and are unable to see themselves as the talented, pretty person that they are. Does this excuse their rudeness? NO! This behavior puts them on the short list of having friends, real friends. Another of my dear friends, while we were commiserating about this other persons behavior, exclaimed, "they are our spiritual sandpaper! But, who is theirs?" I think it is true that there will always be someone in your life who is your "spiritual sandpaper" but where do you draw the line? When do you say, enough? Am I doing them a favor by letting them treat not just me but others this way? I'm going to do my best to be understanding yet speak up if the door is opened for me to do so. Will I blast this person... no, I will treat and talk to them the way I want to be treated and I will keep praying for them that they will have the heart to understand and the ears to hear.
Many thanks to the 2 friends who "Opened my eyes" by showing me that I wasn't imagining the behavior and another helping me see another reason for the behavior!