This was awesome of you-

If I can pick part of this up like I am prone to do... You are dead-on about what these relationships usually require. It does take someone who self-validates and has 'thick skin'. In your original post's description, that was exactly accurate to what I've learned and experienced. You are also right about when a person slips into codependency it can be extremely damaging in these relationships.

That said, I think it absolutely takes 'sensitive' people. I don't think sensitivity is a weakness, but strength. I believe we are thinking of two different kinds of sensitivity, but for myself I am and need to be exceptionally sensitive toward others. It lends itself to empathy and to seeing through another's perspective. A sensitive person can be strong and these relationships can feel like the partner is 'forcing' them to 'react' codependent. Now that isn't true, but it can feel that way and I've experienced demands directly to that effect. I had to learn to say no and build boundaries, but I need to be sensitive to her and even my own feelings (they can be hard to hold onto in the storm, but they guide me) without needing her validation or letting her harm me. I think 'sensitive' people are less likely to be freaked out or confused in emotional volatility (anxiety attacks, emotional breakdowns.. and better at translating and recognizing abuse).

Basically, I don't believe there is any such thing as 'codependent people', I believe that a person can lead themselves into codependency and feel led there in these and other relationships. But codependent is a temporary state (often returned to) rather than a diagnosis. Yet these relationships require the SO to empathize and validate feelings without validating some behavior. To filter what they accept, not falter or defend 'the truth', and self-validate and build strong boundaries when necessary. A person who finds their self codependent likely has as good a chance at noticing and doing something about it (self improvement / change / therapy) as a BPD sufferer at finding improvement through therapy. Both are hard, but both people are under construction rather than finished with their development.

It is parallel to the claims in the lovedones sub and here when an SO hears that it just didn't/couldn't work because the other person was X way... It is sometimes true, but I believe everyone has a choice to dig their heels in and work on themselves and improve. You are insightful into what it requires of an SO, but I don't believe any 'diagnosis' alone can be used to disqualify or be blamed for why a past, present, or future relationship failed. The failure is usually in being static and unmoving.