You're probably glancing at the rest of the plot analysis thinking that maybe Kafka's skipped ahead a couple of stages. Being a disgusting insect could very well qualify for the "Frustration," "Nightmare," or even "Destruction or Death Wish Stage," depending on how much you hate bugs. But no, Kafka's story begins with the main character suffering a horrendously tragic fate. Let the hijinks begin.
Despite Gregor's intolerable situation, he actually seems to enjoy exploring his new body's capabilities sometimes. Who wouldn't enjoy defying gravity and clambering all around the room? He even feels a light euphoria as he hangs upside down from the ceiling.
Even though Gregor can find some positive aspects to his transformation, his family can't accept any of it – they barely tolerate him. Grete, for example, no longer takes care of his room with the same considerate attention that she did in the beginning. Gregor grows increasingly shabby and famished, and his room more cluttered with refuse, as the story goes on.
Gregor's situation is v aggravated by the fact that his family refuses to believe that he can understand them or that he's attempting to communicate with them through non-verbal ways. His attempt to reach out to his sister during her violin recital only confirms the family's worst fears about what he's become – a despicable vermin.
For Gregor, his family's rejection of him is so absolute that it arguably takes away his will to live. Grete's assertion that Gregor is no longer Gregor, but just a nameless bug that must be eliminated just like any other bug, is a denial of Gregor's existence that Gregor finds so compelling that he agrees with her and literally ceases to exist.