And thus, "more natural" than, let's say, procedural programming.
Animals can be modeled nicely with objects/classes. Physical objects do have internal private state and public interfaces. Also in nature, usually, you have multiple similar objects that work the same but have different internal state.
But even if you're not inspired by biology, it might just come up "naturally". Separating your data into structs, then operations on those structs, and not wanting to deal with all kinds of bookkeeping all the time, letting things manage their own - separation of concerns, aka single responsibility principle.
I think automatic memory management might be a big example of this. Who wants to bother with this malloc/free stuff all the time? Who wants to keep track of ownership everywhere? Instead, have a dedicated subsystem handle it. That allows a higher signal to noise ratio in all the rest of the code that would otherwise be polluted with malloc/free. It also removes the entire class of bugs that can happen when you inevitably forget either of those.
But having memory management in and of itself isn't "OOP". It's just an example of one of the principles that people tout when they talk of "OOP". Those principles can and do apply broadly.
Was Alan Kay thinking of all this? I really don't know. All I know is that the dude digs biology.
https://lists.squeakfoundation.org/pipermail/squeak-dev/1998...
It's a concept I really like and use quite a bit in game development at the inter/intra-entity communication level even if below that I'd pick a more performant substrate.
https://archive.org/details/byte-magazine-1981-08/page/n75/m...
"Design Principles Behind Smalltalk" Byte Magazine 1981
https://archive.org/details/byte-magazine-1981-08/page/n299/...