Durkheim sees religion as society worshipping itself because he is a functionalist. Functionalism is one of the three main theoretical perspectives in sociology. Durkheim viewed religion through a functionalist lens.
Functionalists believe that institutions exist in societies if and only if they help to maintain the cohesion and the stability of the society. We have families, for example, because they give us a way to raise children to be good members of our society. In this view, religion must also exist because it makes us more likely to act in ways that help our society to survive. The purpose of religion, then, is to help our society remain cohesive and stable and, thereby, to help it continue.
If this is the role that religion plays in our society, we can see why religion would consist of society worshipping itself. If religion is going to perpetuate our society, it has to celebrate the main values that our society holds dear. The religion of a capitalist, individualistic society like our own, for example, has to emphasize hard work and individual responsibility. If it does not, it will tend to tear our society apart. Therefore, religion will worship the things that our society approves of. When religion tells us to worship the major values of our society, it is in essence telling us to worship society itself.