In many cases, the targets of narcissists et al volunteer for this thankless position hoping things will magically change someday. This is especially true of people who have codependency issues.
Common rationalizations include: She’ll grow out of it once she becomes a mother. He had such a tough childhood. She just has low self-esteem. She’ll become more secure once she learns to trust me and experiences love. All of his exes were abusive. He just needed to be with someone who really appreciates him. She doesn’t mean the stuff she says when she’s angry. That was the alcohol talking. She’s just stressed about the wedding. She’ll feel more secure once I propose. I just need to be more patient, more loving, make more money, take on even more financial responsibility and not have appropriate emotional responses to being abused and he or she will treat me better.
Many codependents are aware of the disordered person’s issues going into the relationship. However, they believe they can help, rescue, save or love the narcissist, borderline or psychopath into becoming an emotionally mature adult who is capable of reciprocating love, kindness, generosity and respect.
This doesn’t happen. Ever. You’ll have just as much success trying to change a rattlesnake into a golden retriever.