The following quotes speak to the theme of alienation in Kafka's The Metamorphosis:
Constantly sees new faces, no relationships that last or get more intimate.
This quote speaks to the fact that no matter how many people Gregor comes in contact with, he is unable to create any type of relationship at all. He will forever be alienated from all others.
[H]is hand pressed over his open mouth slowly backing away, as if repulsed by an invisible, unrelenting force.
This quote recognizes the fact that people react to Gregor with an almost uncontrolled force to get away from him.
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.
Here, one can see the fact that Gregor has become dehumanized by the fact that his room has become a garbage can. No longer considered human, Gregor is trash like the rubbish thrown into his room.
All of the quotes above point to the fact that Gregor lives, through his hand or not, a life defined through alienation.
Fairly early in the story, Gregor manages to get himself out of bed after the attorney comes from his office to inquire after him. Up to this point in the story, Gregor has repeatedly railed against the injustice of his work situation and reflected on his distaste for travel, for his boss and for his co-workers.
When the attorney arrives and accuses Gregor of a poor performance at work, Gregor gets out of bed and makes an attempt to open the bedroom door.
"He really wanted to let himself be seen by and to speak with the attorney. He was eager to see what the others, who were making such demands of him, would say once they caught a glimpse of him."
This episode points to Gregor's wish to be seen and to be seen for who he is - in all of his pathos - as a person who is suffering but willing to work through the suffering (according to his own take on his character).
Unseen and under-appreciated, Gregor is alienated from his family and his work associates. Despite his efforts to be seen and to clarify his position, Gregor is unable to open the door. The attorney does not understand Gregor's speech.
"All right, people did not understand his words any more, although they seemed clear enough to him, clearer than before..."
Alone in comprehending himself, Gregor is alienated from those around him even before they see his transformation. In this episode, everyone still believes that Gregor is merely ill (or, equally likely, merely lazy). His physical appearance is not yet responsible for his alienation or his treatment by others.
As the story moves on, Gregor's isolation increases. He is distanced from his sister, the one person who made a real effort to connect with Gregor and to understand him. But we can see from these early passages that Gregor's alienation is present at the story's outset.