Why is it long term marriages are soemtimes great and some are contentious?

There are stages in a marriage. Some couples get stuck at one of the stages and never move on to the next one.

Stage 1 - The honeymoon stage (no one ever gets stuck in this stage, unfortunately)

Stage 2 - The fighting stage, where each person tries to get the other person change

Stage 3 - The compromising stage, where each person gives up trying to change the other, and tries working together

Stage 4 - The disillusionment stage, where they give up, but stay together

Stage 5 - The acceptance stage. Where they no longer try to "solve" the problems of the relationship.. . they accept the other for who they are. And there are no more problems.

Stage 5 takes 15-20 years to get to. Although I suppose some people are wise enough to realize early on that you either accept the other or you lose the relationship.

We have unrealistic expectations for marriage:

We think the romance and fireworks are "love".

We think the other person is supposed to "complete" us and "make" us happy.

We think the relationship would be a good one if the other person never did anything that bothers us.

These are all unrealistic expectations, and they are actual barriers to discovering and creating mature companionate love.

Successful marriage happen when two people are happy regardless of the relationship, genuinely accept and care about themselves regardless of the relationship, when people can live with uncomfortable emotions inside themselves without freaking our or getting angry, and when they "own" their own emotions (let go of blaming others and making them reponsible for our emotions ... the "you made me do this, made me feel this .. etc" attitude).