Why do classes come into conflict, according to Marx?

According to Karl Marx, the groups with similar interests and common objectives form same classes. This formation of classes is based on a historical process, which means that in every society class, identity among the people was present. With the change in modes of production, technology and social relations, these classes became antagonist to each other, thus resulting in class conflict. For instance, in a capitalist society, there is a conflict between the capitalists and the working class. In such a society, both of them are in a direct opposition to each other and hence the class conflict arises between classes.