Being in a long term relationship is a test of a person's ability to compromise. They must be willing to accept that the other person does not always see things their way. As long as both people are willing to make reasonable compromises, they will be able to get along with each other as a couple. The trouble begins when one partner is constantly expected to compromise on what they want in life.
The ability to get along with another person is not always one that people learn. They selfishly believe what they want matters the most. Rather than getting along with their partner, the partner is simply getting along with them. These types of people will either find a person that is content with always giving in, or they will spend their lives going through partners. Not everyone wants to constantly compromise to keep a relationship alive.