It depends...you will always have the few couples who are committed to each other, work together, and communicate well to each other, keeping the relationship intact.
I think some things that are ruining what marriage should be are:
1. lack of communication
2. financial problems
3. issues of past and letting go (i.e. growing up in a broken family)
4. infidelity
5. inability to forgive when mistakes are made
6. religious clashing - when the partners come from different religious backgrounds.
I can probably think of a hundred other things, but these are the big ones that stick out in my mind.