There can be many differences that lead to conflict in a marriage. Of these, there are three that tend to be more important than others. They are:
Money and personal finance. Couples often fight over how to handle their money. Often, one wants to spend on things that the other finds frivolous or wasteful.
Sex. Many conflicts in marriage arise because of differences over how often to have sex or about specific sexual desires on the part of one partner or the other.
Children and child-rearing. Couples often disagree about how to raise their children, with one partner wanting to be more or less strict on a given issue.