the tripartite meanings of being, to the source to rationality, from Lee Braver's A Thing of This World
The point about this question is that no answer could possibly satisfy it, since any answer would have to take place within an epochal understanding: Being itself is “prior to everything. The prior, of which we really can not think . . . because the nature of thinking begins there” (P. 83). However, answers are not what he is after. Being itself as the source of the sendings sounds like an answer but actually functions as a placeholder or even a counter-answer, which is one reason why he takes to crossing it out in some of his later works. “There is nothing else to which propriation reverts, nothing in terms of which it might even be explained” (P. 415); instead, “it can only be experienced . . . as that which grants” (415). In order to experience this, we need a change in attitude or perspective that instills a new way of relating to the world: “If the answer could be given it would consist in a transformation of thinking, not in a propositional statement about a matter at stake” (P. 431; see also pp. 25–26).
What prior to everything else first grants unconcealment is the path on which thinking pursues one thing and perceives it: . . . that presencing presences. The clearing grants first of all the possibility of the path to  presence, and grants the  possible presencing of that presence itself. We must think aletheia, unconcealment, as the clearing that first grants Being and thinking and [mutual interdependence] their presencing to and for each other. (P. 445, bracketed comments added)Whereas explanatory reason can apply to beings within an epochal understanding of Being, we need a profound shift in order to think about Being itself, which is another reason Heidegger needs a new conception of thinking. “What alone is singularly decisive is the experience of that which is not a being and cannot be a being and yet above all raises beings as beings unto the openness of its sway” (P. 333). The proper attitude to take toward these sendings is not to analyze them for an inner logic, but to respect their “inexhaustible mystery” (P. 64; see also p. 238) by resisting our natural tendency to explain and control: “We never know a mystery by unveiling or analyzing it to death, but only in such a way that we preserve the mystery as mystery.”[P. 43; see also p. 45; p. 51, 93; p. 56; pp. 136, 448; p. 310; p. 13.]
Although technological thinking tries to capture everything there is through comprehension and explanation, the source of rationality cannot be grasped this way. Heidegger rejects Hegel’s attempt to order and comprehend history and reason, claiming instead that the changing of an epoch or “the surmounting of a destining of Being . . . each time comes to pass out of the arrival of another destining, a destining that does not allow itself either to be logically and historiographically predicted or to be metaphysically construed as a sequence belonging to a process of history.”[P. 39, also 44; p. 108; 335; 115, 180–81; 30; p. 130] All that we have is the Unmoored proliferation of epochs of Being, not a rational explanation of any one of them, the combination of them, or the fact that they exist at all. Seeing these radically heterogeneous modes of Being together imparts Heraclitus’s insight that eternity is a child playing where “the ‘because’ withers away in the play. The play is without ‘why.’. . . As the abyss it plays the play that, as Geschick, passes being and ground/reason to us.”[P. 113] The important thing is to stay open to the mysterious fact of presence and keep explanations from extinguishing wonder. When explained, “Being is not acknowledged as Being. Such ‘acknowledging’ means allowing Being to reign in all its questionableness... But that means to reflect on  the origin of presencing” (P. 201, bracketed comment added). Since the series of epochs cannot be organized and placed into the service of a transcendent goal, they can only be viewed as “the ever playful jointure of never resting transformation” (P. 320). We can see premonitions of Derrida here.