What men have called friendship is only a social arrangement, a mutual adjustment of interests, an interchange of services given and received; it is, in sum, simply a business from which those involved propose to derive a steady profit for their own self-love.

This quote is saying that what people usually call “friendship” is not really a true friendship. A true friendship is special and unique and involves more than just “profit”. Instead, this quote suggests that people are often in friendships for their own gain. They agree to do things for each other because they think they will benefit in some way. They are not doing kind things out of the kindness of their hearts, they are doing them in order to get something back in return.