We are not isolated from the thoughts and feelings of our partners, even if those thoughts and feelings are directed at another person, their job, their family, or hurricanes in Malaysia.
Sure, our partners might have issues in another relationship and those issues aren't about us and our relationship with our partners. But few people are able to completely isolate their feelings in one relationship from their other relationships. It's a perfectly reasonable question to pose: "Given that I cannot control whom she associates with, how much carry-over pain and angst am I willing to put up with? At what point do I say enough is enough, and move on?"
Allowing someone to continually bring negative energy home and into our own relationship is not healthy for ourselves or for our relationship to that person. It sucks our energy as that person continually requires our support. As much as it's unhealthy to control other people, it's unhealthy to tolerate our own lives being dominated by the actions of people we have no direct contact with.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker