The Alter Rebbe (Rabbi Schneuer Zalman of Liadi, 1745-1812) explains that the cause of machloikes (disputes or quarrels) is the narrowmindedness of being unable to tolerate another’s perspective; achdus (unity) comes from broadmindedness, the ability to accommodate two approaches to an issue.
Elsewhere, the Alter Rebbe writes that machloikes comes about because of a negative spirit that enters one’s heart, causing him not to trust his friend, despite that person’s friendship. Another cause of machloikes: Instead of focusing on quarreling with himself in order to overcome his yetzer hara, a person diverts that energy to quarreling with others.