What is one main reason some governments in the twentieth century began campaigns of genocide?

One thing all governments have in common, is the act of attaining and upholding their ability to wield power, however they may do so. Governments in the 20th century typically rewarded supporters while rooting out any opponents, making them "disappear". The campaigns of genocide was to eliminate as much, if not all, opposition against the government, while rewarding supporters. Anybody on the fence on whether to support or resist, would see the results, and as a result, would themselves accept or support the government, whether whole-heartedly or reluctantly.

With power, a group can attain control, and with control, power.

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intimacy, passion and commitment are the components of a interpersonal relationship emphasized in the triangular theory of love by robert sternberg.

the triangular theory of love says love can be comprehend as this componest form the vertices of a triangle.  

intimacy relates to feelings such as of closeness and connection;

passion relates the drives that brings people together, as physical attraction.

commitment relates to the being in a long-term relationship.