The most difficult adjustment is for humans is marriage. Because as humans we place demands on each other that is unreasonable and it can also be the same in marriage. In unhealthy relationships opposite characteristics and traits attract and often time do not get along.
Two kinds of dysfunctions occur in these struggling relationships you either have a dominant male or a dominant female and the mate is passive. There are studies that show that by the age of 20 humans tend to have developed their own identity and are independent in their thinking which sometimes creates an inability to change. So whatever weaknesses they posses must be matched by strengths in their mates. A person that is dominate will tend to find a mate that is compliant in different ways to them for example introverts are attracted to extroverts, emotional person will be attracted to someone that is a more cautious thinker, a non-talker to a person that can't stop talking.
After marriage what happens is these traits becomes faults for each person and often times what you have is criticism and resentment of each other weaknesses. When you ask these couples why did they get married if you knew this about your mate, you will often hear that they thought that they could change each other. Wrong answer...
When two people are attracted to each other their is an anticipation that there will be happiness in a relationship but often time what you find in relationship is issues of power. Couples need to understand that marriage is a co-dependant relationship where two people share and fulfill each others needs. It is a relationship that is built on support and partnership not to complete each other's weaknesses.