The notion that the ethical is the universal is the disclosed and the aesthetic is the single individual is the hidden originates in Hegelian philosophy of language, which can be explained as follows. We learn language in a public environment from other people and we use language to express thoughts to other people. Thus, language is a purely public phenomenon. It can only be used to express experiences that we share, which we can understand in common. If I want to tell you of an experience I've had and you haven't, I must do so by comparing it to other experiences you might understand. For instance, I might say that reading Kierkegaard is like being forcefully tied to a chair and then being made to watch a fireworks display. You may have not read Kierkegaard, but you might have been tied to a chair and you might have seen a fireworks display, so this use of language might bring you closer to understanding an experience you yourself haven't had. Language is thus related to the universal, since it is useful only insofar as it concerns itself with experiences we can share.
Since the ethical and the universal are associated, it is natural that the disclosed should also be associated with the ethical. Since the aesthetic deals with the private experiences of the single individual, it similarly makes sense that it should be hidden: language cannot penetrate these private experiences. Johannes' examples all deal with instances in which people are faced with the decision either to speak and cause great dismay, or to keep their unhappy knowledge a secret. The aesthetic sides with secrecy, the ethical with disclosure.
The motivation in each case for the aesthetic hero is to protect those he cares about: the bridegroom will cause his future wife great dismay if he tells her that their marriage will be his undoing. The aesthetic hero takes the responsibility for the unhappy knowledge fully upon himself, so as not to burden anyone else with it. In doing so, he isolates himself as a single individual, and is thus isolated from the ethical. He is doing what he thinks is right, but he is acting privately as a single individual, and is thus trying to enter, as a single individual, into a relation with the absolute.
The ethical, or tragic, hero is motivated instead by the desire to be totally open about everything. The possibility is ever present that the aesthetic hero is really taking on the burden of secrecy to protect or comfort himself. This is not true for the tragic hero, since he keeps nothing secret. In surrendering entirely to the universal, the tragic hero also surrenders the pretension that, as an individual, he can save or protect anyone by his silence.
In each example, Johannes presents us with a choice according to which the hero would define himself either as an aesthetic hero or a tragic hero. A central theme in Kierkegaard's thought (which proved to be tremendously influential on existentialism) is the idea that one is fully responsible for one's decisions, and that certain decisions are fundamentally definitional of the kind of person one is.
The tension for the aesthetic hero is that he is always free to speak up. His secrecy violates the demands of the ethical, and if he were to explain himself, he could easily be understood. He remains hidden by choice, not by necessity. In this way, the aesthetic hero is distinguished from the knight of faith, who has no choice but to remain hidden. The knight of faith cannot speak because he cannot be understood (this will be explained in more detail in the sections to come). Both the aesthetic and the religious deal with the single individual in isolation from the universal, but the aesthetic deals with the single individual who isolates himself in an attempt to relate to the ethical as a single individual, whereas the religious deals with the single individual whose faith brings him above the ethical.