Sometimes I think of this in terms of the Roman god Janus. Only instead of having one face looking into the past and one into the future, we have one face looking inside ourselves and one looking outside, at the world. One introverted, one extroverted.
While maybe not the case for everyone, perhaps for some people with a different internal self-image from our external physical one, when something happens which makes one intimately aware of oneself on a physical level, the introverted face wakes up, and all its attention is focused on the personal. The visualisation of the self-image. Like a ray of sun through a magnifying glass. It becomies acutely aware of every nuance of emotion, every feeling of discomfort. This face sees into the darkest recesses of ones mind and shows all the things wished for, illuminating the self-image very strongly, and the dysphoria and discomfort of not matching that reflection of the psyche on the outside. Often to the point where the extroverted face begins to fall asleep and the dysphoria is all one can think about.
Conversely, when not doing something which draws attention to the disparity of ones physical sensations with our mental self-image, the extroverted face is wide awake, observing and interacting with the world. At those times the introverted face sleeps. And life becomes a constant shift in awareness from one to the other.
Perhaps for some, the choice to take the first steps in transition (which is partly what the god Janus represents), is reached when both of these faces see the same thing, come to the same conclusion, and agree on a course of action.