The Fixer-Upper

We've long established that there's no therapists in Fictionland, or at least no one with the requisite income or whose job includes health insurance. For a surprising number of characters, this doesn't seem to be a problem: just find a love interest (usually female) and they'll patch you right up! No matter what a disaster zone the character is, or how poorly they treat their partner, all will be solved and forgiven by the power of True Love.

In Fictionland, the damaged character is a fixer-upper, just waiting for the right person to come along and make it all better. Now, I generally believe that good relationships change the participants for the better, even if the relationship ultimately doesn't last. And we lean on our partners when the going gets tough, often with a greater intensity that we lean on our other support systems. But the Fictionland version goes beyond a deeply supportive partnership and borders on one character being a wreck, whilst the other takes the job of cleaning up after them.

Obviously, this dynamic exists far too often in real life. People of all genders, ages, cultures, sexual orientations, etc. get tangled up in unhealthy relationships for all kinds of reasons. If your story shows the relationship as being dysfunctional-- that is, the reader is supposed to watch the couple's interactions and cringe-- that's one thing.  My peeve is about authors who write about what is, bluntly, a toxic relationship, and either normalise or romanticise the behaviour they describe. There are three main messages here:

True love/TLC/general doormat behaviour fixes the jerkass partner. Implied: if the victim showers the abuser with affection and they're still behaving abominably, the victim didn't try hard enough, and the behaviour is thus the victim's fault.  This trope hits women particularly hard because of the Victorian-era concept that women were naturally nurturing, and thus meant to reform and 'civilise' their naturally depraved male partners.The scary/controlling/manipulative/violent behaviour means they are just super-duper in love! The message here is that whatever awful thing the person is doing is actually a romantic gesture (seriously, what happened to candlelit dinners and long walks on the beach?) and the victim should return the affection.The dysfunctional couple is normal/healthy/totally in love. This tells us that it's normal (and even good) for a relationship to include partners stalking, spying, getting cut off from friends/family/support systems, screaming rage, etc. Not exactly the narrative we want for our society, especially given the disturbing percentage of youngsters for whom fictional media is their  main purveyor of sex ed/relationship advice. 
I'm not telling you not to write characters whose romantic relationships are a horrific mess. Just don't write a horrific mess and call it tender romance.