The main reason for domestic or dating violence is

Domestic violence — also known as domestic abuse, intimate partner violence or abuse — may start when one partner feels the need to control and dominate the other. ... Studies suggest that violent behavior often is caused by an interaction of situational and individual factors.Some people who experience depression or other mood disorders are often more likely to be victimized. Substance abuse: People who abuse drugs or alcohol may fall victim to someone who is abusive. A victim's need for acceptance or money to support their habit may cause them to be vulnerable to domestic abuse.

Explanation:

They love each other so much it hurts

Most people assume it's because of violent games or something. However many people believe it's because the person grew up with parents who where also in a domestic violent relationship and soon developed problems of their own. They don't know how to react to their partner or don't know how to treat them because they're never seen how to properly treat someone.