American Psycho: Why does Patrick Bateman keep randomly changing the subject in his narration?

I just started reading the book recently after seeing the movie countless times and quite honestly it quickly became one of my favorite books.

So I got the part where he went to dinner with Courtney and some couple he found annoying. So he was talking about how the girl wanted a diet coke but her boyfriend kept trying to convince her to drink alcohol. So at some point she says she wants rum in the coke. But Bateman suggests she should have Pepsi instead. Then he says everyone looked at him disapprovingly.

And then, out of nowhere, he gives a long detailed description of what he saw in some threesome porn video he rented. After that, he goes on about some Stephen Bishop record he bought. Then he takes the reader back to the dinner scene and the conversation about Pepsi and Diet Coke.

So, without spoiling the rest of the book, why does Bateman do this?