The self, in its efforts at self-realization, reaches out beyond the ego-personality on all sides; because of its all-encompassing nature it is brighter and darker than the ego, and accordingly confronts it with problems which it would like to avoid. Either one's moral courage fails, or one's insight, or both, until in the end fate decides. The ego never lacks moral and rational counterarguments, which one cannot and should not set aside so long as it is possible to hold on to them. For you only feel yourself on the right road when the conflicts of duty seem to have resolved themselves, and you have become the victim of a decision made over your head or in defiance of the heart. From this we can see the numinous power of the self, which can hardly be experienced in any other way. For this reason the experience of the self is always a defeat for the ego.
— Carl Gustav Jung
Jung is describing something the ego will not voluntarily admit: that the self does not arrive as illumination but as a kind of ambush. The ego marshals its arguments — moral, rational, perfectly reasonable — and those arguments are not wrong. Jung is careful to say so. You should hold them as long as holding is possible. The problem is that holding becomes, at a certain point, a form of avoidance dressed as integrity, and the soul knows the difference even when the ego does not.
What makes this passage worth sitting with is the word "defeat." Not transformation, not integration, not the language of growth — defeat. The ego loses something it cannot recover, and that loss is the only reliable sign that something real has occurred. Every other version of the self's arrival — the epiphany, the breakthrough, the moment of clarity — can be manufactured by the pneumatic appetite, the hunger for ascent that passes suffering off as transcendence. The genuine encounter cannot be manufactured because no one would choose it. You become, as Jung says, the victim of a decision made over your head. The numinous announces itself precisely by the absence of the ego's consent.
Carl Gustav Jung·Mysterium Coniunctionis: An Inquiry into the Separation and Synthesis of Psychic Opposites in Alchemy·1955