People want to stand for something, which means opposing those who stand for something else. In the course of opposing they will often resort to insu… - Harvey Mansfield

The notion of thumos tells us further that politics is about protection, not primarily about gain. The reason you assert in your defense protects you and people like you that are included in the argument you advance. In an assertive, political argument you assume that you are perfectly OK. You are not apologizing for your self or your soul. The problem lies in things outside you, accidents that have happened or might happen, or the faults of others besides yourself. You therefore want to be protected in your self-satisfaction.

The simplified notion of self-interest used by our political and social science cannot tolerate the tension between one’s own and the good, for that tension leaves human behavior unpredictable. One cannot penetrate into every individual’s private thoughts, and there is no clear way to judge among different conceptions of the good. So in order to overcome the tension, science tries to combine one’s own and the good in such a way as to preserve neither. It generalizes one’s own as the interest of an average or, better to say, predictable individual who lives his life quantifiably so as to make its study easier for the social scientist. And for the same purpose it vulgarizes the good by eliminating the high and the mighty in our souls (not to mention the low and vicious), transforming our aspiration to nobility and truth into personal preferences of whose value science is incognizant, to which it is indifferent.

Self-interest, when simple, is universal; I would do the same as you. I would be propelled toward an obvious good, or toward a good I thought obvious. If self-interest is obvious, it is not really your very own; it has been generalized, perhaps artificially.