What is striking about the first chapter of this novella is that Gregor wakes up one morning to realize that he is changed into a "monstrous vermin" and yet he doesn't seem all that bothered by the fact. He is frustrated over his inability to get used to his new body, but he never asks himself how this happened, why it happened, or what he could do to return to his normal self. There is no textual answer to your question, but you need to infer from his initial reaction and lack thereof that Gregor thinks that he is merely changed for the moment and that he will return to normal soon enough, and in the meantime he doesn't want to alarm anyone. He seems to think that if he can just get out of bed he can get himself together enough to go to work like normal, even if he will be a little late. Gregor is also extremely sensitive to the sensibilities of his family members and he doesn't want to alarm anyone else with his changed self. He sacrifices himself in large and small ways to please his family, and this choice to not ask for help is just another example of that behavior.