First person narrative is very late literary development in the history of the West, allowing more psychological complexity even than the psychological novel (also very late). It doesn't just presume inner complexity like the novel, it gives actual access to interiority.

It also presumes a certain topography of interiority. It shows the processes of doubting, deliberation, difficult choices, but also of "bad but understandable" behaviours - it lets you merge with and understand a character who nevertheless wants to do something that might in some exterior philosophical framework or worldview be judged as bad, like killing out of revenge or yearning to fuck a preteen. It presumes psychological complexity in a way that more ancient narratives never did, because they judged actions, not motivations. It presumes that motivations exist and are complex. It presumes that we can sympathise with motivations even when they lead to bad outcomes, or even if they are in themselves wholly bad.

It therefore assumes a lot of things implicitly about the nature of consciousness, choice, action, responsibility, etc. You could trace fundamental changes in the nature of cultural perceptions of identity by showing that post-Freudian (e.g.) narratives tend to assume a subtly post-Freudian structure of mental topography, unconscious drives, etc. Many many other applications.

Tied into the complexity of thought, character, and action that it allows, it also allows us to dissolve our own interiority into the author's. Unreliable narrator is a big thing. You can find yourself swept up in the actual motivations of the author, merging your identity with this and making assumptions ABOUT identity. Because anthropology for instance assumes the psychological and cultural structure of "identity" concepts are incredibly malleable (google that shit), this could be worth thinking about.

I think Ong talks about Dickens and the psychological novel specifically if you want to rip some good famous quotes.