[…] constantly seeing new faces, no relationships that last or get more intimate. (1.4)
Ironically, one of Gregor's main complaints about being a traveling salesman is that he can never establish a lasting relationship with anyone because he's always moving around. Stuck at home over the course of the story, he's still unable to make a connection with anyone.
But still, the others now believed that there was something the matter with him and were ready to help him […] He felt integrated into human society once again and hoped for marvelous, amazing feats from both the doctor and the locksmith, without really distinguishing sharply between them. (1.23)
Before he encounters anybody in Part 1, Gregor can still cling to the hope that he's part of "human society," and thus can be helped or cured by other human beings.
His room, a regular human room, only a little on the small side, lay quiet between the four familiar walls. (1.2)
After the transformation, Gregor takes a look at his room. It seems familiar enough, but the expression a "regular human room," a room for humans and not for vermin, suggests that he's already feeling uncomfortable in his own room.
Well, leaving out the fact that the doors were locked, should he really call for help? In spite of all his miseries, he could not repress a smile at this thought. (1.14)
Even though Gregor hasn't shown off his new body to anybody yet, he already feels more isolated. He's already aware, for example, of the effect that his transformation will have on everybody. He's still able to find some humor in the idea here, though, in contrast to his dejection in Part 3.
Gregor […] complimented himself instead on the precaution that he had adopted on his business trips, of locking all the doors during the night even at home. (1.7)
Gregor's habits on the road become part of his everyday life. Locking up his room in his own house suggests that he feels his home to be just as unhomely as a hotel.
The Office Manager
[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.
That's all I'd have to try with my boss; I'd be fired on the spot. Anyway, who knows if that wouldn't be a very good thing for me. If I didn't hold back for my parents' sake, I would have quit long ago. (1.5)
The "that" that Gregor is referring to here is waking up late like the other traveling salesmen. Ironically, that's just what he's doing here, although we don't find out he's late until the next paragraph. Of course, he has a very good reason to be late – he's a vermin. But the transformation has the unintended consequence of fulfilling Gregor's wish to be rid of his job.
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.
He thought back on his family with deep emotion and love. His conviction that he would have to disappear was, if possible, even firmer than his sister's. (3.29)
Gregor's desire to disappear is quite understandable given everything that's happened. The word "disappear" is even more absolute than death, which would at least leave a corpse; perhaps Gregor doesn't want to trouble his family with the burden of getting rid of a conspicuously large bug. Inexplicably, Gregor continues to have "deep emotion and love" for his family, despite the fact that they really don't think of him as Gregor anymore.
[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.