At the beginning of a relationship - in the loved up state - all we look for - in fact all we see - are our similarities. Usually, just around a two year period, we start seeing the differences and wonder where they came from.
We also put on our 'best' face in the beginning when we should just let people see who we really are. If you hate football, tell him, don't pretend that you love it.
We don't ask each other enough important life questions, and so go into marriage semi-blind. It's a huge shock then when, for instance, the wife wants to nurture the children and husband believe in tough love. Just in this one area, there is enough conflict to divide.
We are also in a throw away society and no longer seem to believe that we need to work at a relationship to at least see if we can make it succeed. We give up at the first sign of trouble and never really find out how the other person is truly feeling.