In the second half of Your Name (after the body switching stops happening), Mitsuha and Taki can't remember each other's names. They can even end up forgetting the names within 10 seconds of seeing each other. Why does this happen? One theory I have is:
Before Taki intervenes, Mitsuha would have died when the comet hit. Taki couldn't remember her name because her future was hanging in the balance.
But this answer doesn't entirely work. For one,
He still fails to remember her name after they rescue Itomori, and Mitsuha is no longer at risk of dying
And second off
Taki was never in risk of dying in the comet, but Mitsuha still can't remember his name either.
So why did this happen? I know this is somewhat the main theme of the whole movie, but it never made sense to me how they could forget so quickly. It's almost like something was actively causing them to forget.