Endogamy

Endogamy is the idea that a person ought to marry another person from the same group as them. The type of group is not always the same; it could be based on a religion, a class or a social group. Often group endogamy is enforced by arranged marriages, this is a system based on benefits for the group rather than marriage for love.
Group adherence.
By marrying a person with the same group characteristics, it means that the partners will have similar views or beliefs about something. Many people marry endogamously for reasons of compatibility, which can be due to choice. But there are also other reasons when there is less choice involved or there is disapproval if it is not followed.
By doing this it keeps the numbers of a group to a minimum and thus creates stronger group bonds. For example, when moving to another country, a Sikh family may actively look for other Sikh families to mix with in order to facilitate Sikh marriages. By merging, they create a stronger bond between their own communities and can be a way to deal with being a minority.
Biology.
If a group has been marrying endogamously for a long period of time the gene pool may be smaller and there is a greater chance of genetic disease. For example, within the Jewish community Tay-Sachs disease is a common genetic disease.