I think the reason that it's not recognized is because there's not one set cause nor manifestation of symptoms; like how recently they've declassified a bunch of personality disorders into spectrums or clusters, since no one falls firmly into one thing and there's a variety of causes, even though a large majority of them are rooted in insufficient or abusive upbringings.
To me, it's one of two things.. either this person is a jerk, isn't in love with you, just put up with you or whatever and wants out. Or, this person is genuinely having a break down, an identity crisis, or some other form of lapse that most likely stems from them having serious issues they put off dealing with, typically by over focusing on a spouse, career family or some combination of.
If the case is the latter and the person is seeking help and wants to fix things, why shouldn't we let them try? I think in society we're too easy to dismiss those that have stumbled. Too quick to shun those who have cracked under some pressure and make them outcasts and blow them off because that person has "problems". We have a mental health crisis in the US, without a doubt, because we walk past people in need and say "it's none of my business" or "they have issues". Yeah, they do. Don't they deserve understanding, help, patience and kindness as human beings who are suffering?
Off my soap box and I'm not saying it's ok to be a doormat either.. but we do have serious, serious issues with being aware of and discussing mental health issues as s society.