As Upstream mentioned, often there is codependency involved. The partner without PTSD has a strong desire to help, fix, rescue, care for, or even control the PTSD sufferer. It gives them a sense of purpose in life, makes them feel important or needed. Unfortunately, if this is the case and the sufferer does get help for themselves and heals, the relationship inevitably comes to an end, as the non-PTSD partner no longer has someone to help or control. This is why it is so essential for both partners to be in therapy of some sort and heal simultaneously. It cannot be a one way street if the relationship is to survive long term.
This perhaps sounds harsh, however I strongly believe that one attracts, or is attracted to, someone who is rather like themselves, at least as far as mental health is concerned. So, if one is attracted to someone who is very ill, perhaps they have problems and unresolved issues of their own which they need to address. Because truly, if you are a very healthy and happy individual, why would you freely choose to be with someone who is not? It makes no sense.
I would encourage anyone who feels attracted to a very ill person - whether than be an untreated PTSD sufferer, or an abusive person, or an alcoholic/drug addict, etc - to ask themselves very honestly why? Examine your own sense of self-worth and self-respect. Examine why you wish to be with someone who is so ill. Do you feel this is all you deserve, the best you can do? Do you feel a strong need to caretake? Are you repeating a pattern from childhood? Take an honest look at yourself.
It is of course an entirely different matter if your partner develops PTSD after you have been with them for a time, have an established relationship, are married, have children together, and so forth. However I don't believe that is what you are asking about Evie, correct?