My experience tells me there are several common problems couples need to work out.  The list goes like this: (not necessarily in this order) communication and conflict resolution, finances, extended family, sexuality, parenting, friendships and faith.  When couples can be on the same page on most of these issues, they will be more "compatible" and will feel happier.  When major disagreements happen on several of these issues couples won't feel very happy.  When there are differences and couples can find compromises that work for both, they will get along better.  When couples can't seem to compromise, the issues just keep coming back again and again.  Counselling can be a safe place to talk about these concerns and get help to work out solutions.