How we cite our quotes:
[T]he manager burst out with a loud "Oh!" – it sounded like a rush of wind – and now he could see him standing closest to the door, his hand pressed over his open mouth slowly backing away, as if repulsed by an invisible, unrelenting force. (1.25)
While up to this point Gregor's isolation seems to be self-inflicted, the manager's reaction shows how Gregor's isolation is also imposed upon Gregor by the people around him. Before he was turned into a vermin, Gregor chose the lonely life of a traveling salesman, and chose to lock his doors in his own home at night. After the change, it will be the others who lock him up in his room.
But the empty high-ceilinged room in which he was forced to lie flat on the floor made him nervous, without his being able to tell why – since it was, after all, the room in which he had lived for the past five years – and turning half unconsciously and not without a slight feeling of shame, he scuttled under the couch. (2.5)
As the story goes on, Gregor feels increasingly out of place in his own room. Whether the anxiety created by the high ceilings in this particular passage is a function of his anxiety as a human being or some biological imperative of his insect body is unclear. If it's the bug side of him that craves dark, enclosed spaces, Gregor is still human enough to feel shame.
[M]any things had become superfluous, and though they certainly weren't salable, on the other hand they could not just be thrown out. All these things migrated into Gregor's room. Likewise the ash can and the garbage can from the kitchen. Whatever was not being used at the moment was just flung into Gregor's room by the cleaning woman (3.9)
Gregor's isolation is rendered complete when his room is no longer the "regular human room" of Quote #1, but an over-sized storage closet/trashcan. You could say he's an exile in his own home.