Most Helpful Guy
Some people can't live with the idea of being alone because they require companionship and love to get through the day. Others are trapped in the relationship due to financial struggles or someone having too much control in it. A few are so used to the abuse of their partner that anything nicer than they are in would seem unrealistic to them.
People who have low self-esteem, don't set standards or don't do their research on the person they are dating beforehand are more prone to staying in an unhappy relationship. It does seem odd from our point of view why some people stay in unhappy relationships but there is always a reason for it.
Most Helpful Girl
I was abused by my parents so I think I can answer this - when you have low self esteem - you get to a point where you prefer mistreatment to kindness from the person you are dating - you would RATHER be with somebody who mistreats you then somebody who treats you well because you are used to being mistreated. In the same way that alcoholics are used to drinking alcohol even though they know its bad for their health.