The parents may have an objection to their child marring someone "different". Friends may not accept the new person into their circle of friends because of the difference. Although society in general has become more accepting over the past few years, there are many who still see a mixed marriage as a betrayal of their race and are not afraid to express or show their disapproval. Children born of a mixed marriage are more likely to find disapproval from their peers in school and are less confident of where they "belong". Some countries and some races are much less accepting of a mixed marriage than are others.
Different cultures and different religious backgrounds.
Conflicts of interest causing extra needless suffering fullfills some scenarios in the myriad of chaos. Other results of greater good refines everything creating better standard with it all.