I like to think about how Zane was created to be human, when Pixal, Echo, Mr E, and Cryptor (and somewhat Unagami) were all created to be nindroids.
Every nindroid other than Zane was created with the intention of a mechanical being with a conscious, someone who can be compared to human, but is their own species all together. They were never expected nor created with the intention of them being human.
Zane, however, was created intentionally as a mechanical being that was more human than nindroid. He is the one and only nindroid that constantly worries about emotions, humans, being human, fitting in, and his very existence.
(Unagami does not count his whole thing was solely daddy issues cause his dad was a bit closed minded)
Maybe thats why Zane slowly becomes more robotic. As years go by, Dr Julien is no longer there and everyone including Zane and Dr Julien acknowledged his nindroid side to be who he is(that he is not human, and that is okay, because that doesn’t change a thing. But subconsciously, to Zane, it almost changes everything) but Zane never really felt like a nindroid. So as time passes and Zane is destroyed and rebuilt and has more newer pieces for beings who were created as nindroids to live as nindroids, Zane no longer feels like his soul and his body are one in the same.
It feels trapped. Like he can no longer find the distinction between Zane the human and Zane the nindroid, because whilst both were him, it now felt like one was squishing the other. He just wasn’t sure which one.
I don’t think i explained it well so I’ll stop now cause it’s late. Might explain properly later. But basically because Zane’s body is being constantly rebuilt due to situations it’s making his soul mad uncomfy.