There has never been any firm evidence as to why Gregor becomes a giant insect in Kafka's The Metamorphosis, though it has been debated widely.
There are several things that might account for Gregor's change. It could be something rooted in his own mind. In keeping with this argument, it could be a "waking-dream." He notes that he had strange dreams the night before:
One morning, as Gregor Samsa was waking up from anxious dreams, he discovered that in his bed he had been changed into a monstrous verminous bug.
However, Gregor soon dismisses this himself:
“What’s happened to me,” he thought. It was no dream.
That is not to say, however, that he could not be having a "dream within a dream."
It has been argued that Gregor might simply perceive himself as a "verminous bug" because of the way his family thinks of him. Once his father lost the family business, Gregor has slaved so that the rest of his family can live in comfort. He notes that at first it was appreciated. However, as time goes on, it is taken for granted, and we see that once he is unable to continue to work, he is regarded with contempt, particularly by his father. Gregor's desire to help the family is evident from the first:
At the time Gregor’s only concern had been to use everything he had in order to allow his family to forget as quickly as possible the business misfortune which had brought them all into a state of complete hopelessness. And so at that point he had started to work with a special intensity and from a minor assistant had become, almost overnight, a travelling salesman, who naturally had entirely different possibilities for earning money and whose successes at work were converted immediately into the form of cash commissions, which could be set out on the table at home for his astonished and delighted family. Those had been beautiful days, and they had never come back afterwards, at least not with the same splendour, in spite of the fact that Gregor later earned so much money that he was in a position to bear the expenses of the entire family, costs which he, in fact, did bear. They had become quite accustomed to it, both the family and Gregor as well. They took the money with thanks, and he happily surrendered it, but a specialwarmth was no longer present.
His father's lack of tolerance is seen when Gregor comes out of his room—his father throws apples that painfully lodge into his son's back. Grete stops her father, but later, even she turns on her brother.
It is also possible that Gregor feels like a "bug," or an "outcast" because of his job. The company he works for shows him little regard: checking to see if he's lying when he doesn't show for work, rather than showing concern. He also works hard:
“O God,” he thought, “what a demanding job I’ve chosen! Day in, day out, on the road. The stresses of selling are much greater than the actual work going on at head office, and, in addition to that, I still have to cope with the problems of travelling, the worries about train connections, irregular bad food, temporary and constantly changing human relationships, which never come from the heart...”
The fact that Gregor cannot have a relationship that comes "from the heart" indicates that he feels isolated, alienated. Gregor is disconnected from society—always traveling; from his family who see his value in his salary; and, from his coworkers.
Because of the way he is treated, Gregor may be an insect or simply feel like one.
Gregor Samsa is very similar to Franz Kafka. when Franz suffered from insomnia, he thought that he was a hardship for his family, just like Gregor.