To be successful romance writers, we have to understand basic human nature. We have to know how people react to situations and how they deal with things. Along the same lines, we need to know who are characters are. We need to understand their flaws and figure out ways for them to grow as people so their relationships can be healthy and strong.
But what is a healthy relationship? There's no point in denying it--we all have our flaws, especially when it comes to relationships. And although a romance novel should never have perfect characters, we can't have the hero beating the heroine (an extreme example, but I have read a romance novel where the hero has done this). Personally, I was so upset that I refused to buy another book by that author ever again. I don't want to read about a hero or heroine who acts in such a morally unacceptable manner.
So out of curiosity, I googled 10 Warning Signs of a Bad Relationship and came across this really good article that discusses what you should watch out for. I think the points are valid. Basically, it's a list of what NOT to include in a romance novel. You should check it out and let me know what you think!