It may be an identity crisis. Many twins feel that they have to work much harder to be unique because they may constantly be compared to the other counter part. This can lead to negative and poor relationship between twins as well as parents. This article sheds some light on why twins tend to live very different lives later on. "The causes of these differences were due to changes in the human epigenome, he realized." Doctor Spector found this research and further stated, "pigenetic changes are not just simple environmental changes, however. They influence a person's genes and can have an effect that can last for two or three generations in extreme cases."
If a twins get married first, the other may be left grieving. I may not completely agree with all that is stated here, but I do believe that the connection twins share is like no other.