why was Lisa turning like everyone else ?

Well we are seeing the world from Michaels perspective. So in the real world everyone doesn't actually look and sound the same. Michael sees something different in Lisa and that's why she sounds different she's the anomaly. But then Michael starts seeing her like everyone else, she talks about the zoo and that triggers it. Think that's a nod to the tax driver talking about the same thing. So Lisa hasn't actually changed in the end of the movie, but Michael's view of her has.

the thing is that Lisa fits perfectly to the object-type person he want's to engage with. By being vulnerable, he finds himself in an advantageous position with her, even though that might not be clear for him.

Also, she is constantly repressing her desires and preferences. I think that appeals even more to Michael. By the way, i think the reason he wouldn't choose Em over Lisa is because was interested in him. He doesn't want somebody who has interests. (It's true that Em's voice was like everybody else's from the beginning, but i think the film is arguing that we can sense this kind of thing from the beginning. We can sense who might fit our expectancies from the first moment)

it's when Lisa starts to show herself as a person that Michal gets troubled.