this is a difficult one to answer since we're not as intensely therian as some friends, but i'll try
we've had sensations of phantom limbs (my wings, either mine or π's tail) for years. they're intermittent, but we do sometimes need to reposition in bed to account for the wings, as one example.
what pushed us over the edge from just "we have phantom limbs" to "we're therian" was the intense nightmare we had in february; one of the aftershocks for hours afterwards was that the wings attached to us were wrong and the amount of distress that caused.
(π©· is part of the system now and it isn't a big deal if their wings pop in now and then, but on that day it was).
beyond that, there's the draconic instinct to hoard and collect, which i sometimes need to fight - and other things, like the thought of tracing claws down a friend's neck
but i don't think any of this is necessarily needed - for some, simply answering the question "if given the choice to be freely whatever or whoever you wanted, would you still be human?" can be enough to claim the therian label
(and our answer to that has been that we would prefer to be non-human as far back as we can remember)
but like plurality, it just took a long time for us to accept that it was applicable
