The majority of people grow up with parents who have a good relationship, and this often prevents them from realizing their partner has serious relationship issues. While some issues are easy enough to solve, many are not. People are often unwilling to change basic patterns of living, and those who are used to bad relationship habits see nothing wrong with their methods. One reason these issues are difficult to spot is because a partner who uses them has learned how to disguise them.
Partners with bad relationship skills will often hurt other people, but they will diminish these hurts as if the person is complaining about nothing. This is a serious red flag in a relationship, but there are others. An explanation that a person needs to be hurt to make them a better person is often an excuse for bad behavior, and it should be a signal the relationship needs to end. Refusal to say why a person is angry is a passive yet aggressive way of handling a situation and is yet another sign it is time to leave.