Kakuriyo: the cost of the hidden realm.
Grandma did warn ‘em…
Remember what she said when Taki as Mitsuha arrived at the Gods home? That it was kakuriyo, the hidden realm; a land not of the human. A place of magic. She also said that to be able to leave from there, you HAD TO leave something precious for you behind. This is why they were leaving the Kuchikamizake.
The thing is- Mitsuha left that. Taki, inside Mitsuha, left nothing. The Kuchikamizake was part of Mitsuha, not Taki.
He owed the God/Kami.
The Kakuriyo tried to resolve the situation by taking what it considered important to Taki- his connection to Mitsuha. How can we tell?
From there on, Taki could not dream/swamp/communicate with Mitsuha anymore.
The second time Taki is inside kakuriyo, he was able to leave something behind. He had been given a clue in the vision…
By giving Mitsuha the braid, he acknowledge that it was deemed important for him, along with his memories of her. Perhaps that’s why Taki asked Mitsuha to write her name in his palm? Maybe he was aware that he would forget her in consequence of going out of kakuriyo.
He almost made it.
As for Mitsuha?
Her second visit to kakuriyo (not the cave but the power of kataware doki) she had to give something up. She didn’t. So the kami took from her what she considered most valuable… her memories of Taki. The process was slower but it happened, leaving her unable to remember him…
Don’t fuck with kami is all I am saying…