In the Gorgias Socrates argues that what we want is not necessarily what is objectively in our interest, but what we do is what we (often incorrectly) believe is in our best interest, wrong as we may be concerning whatever that may be. In fact, if we knew what was objectively in our interest, that is what we would want and do, rather than what we actually end up doing. Socrates claim is that no one actually wants what is evil. This is called Ethical Egoism. Brickhouse and Smith point out that Ethical Egoism, which many have identified with Socrates, fails the common sense test. First of all, just because something promotes our interests does not mean it is not evil. Just because something comes in the way of us fulfilling our interests does not mean it is evil. Socrates fails to make this distinction between moral and prudential goods.
In order to believe that we can be mistaken about what is in our interest, Socrates is presupposing that what is in our interest is objective. A ‘giddy moron’ may enjoy behaving foolishly.
This becomes more plausible when we consider that Socrates is taking into account instrumental aims. X is not immediately in my interest, but I do X anyways because it allows me to achieve my higher goals. Once I learn that the pain of surgery is actually in my interest, I am willing to go under the knife in order return to health. The tyrant kills and dominates nor for its own sake, but because he believes it will fulfill his higher desires of being safe and protected. He is mistaken that this will actually serve his interests, but this allows to see how the tyrant could possibly behave in this way if we accept the premises of Ethical Egoism.
What is objectively in our interests are those things that lead to our personal eudaimonia. Socrates believes, therefore, that all of our voluntary actions aim at our happiness, and that eudaimonia is the measure by which we determine whether an action was objectively in our interest or not.
On a related note, we see why Socrates is called an Intellectualist, because of his arguments that our actions and motivations are based on our intellectual beliefs. Therefore, all desires are rational desires. This, then, becomes a problem when we read the Apology, in which the picture Socrates offers of the role of our appetites and passions conflicts with this position.