Gregor Samsa is transformed into a giant insect because he has already transformed, mentally and emotionally, into such an insect. Gregor works as a traveling salesman, and it seems that work is literally the only thing he does, just like an insect. His mother even confirms this when the manager comes to Gregor's apartment. She says, "It nearly makes me cross the way he never goes out in the evenings," and she explains that he's been home for a week and yet has not gone out at night even one time. In short, Gregor has no life.
Further, Gregor hates how "strenuous" his job is, how it seems to take everything out of him and provide him little satisfaction at all, beyond being able to pay back his parents' debt. He has to wake up way too early every day, worry about missing his train, eat "bad and irregular food," and make "contact with different people all the time so that [he] can never get to know anyone" or have any real relationships. He doesn't even have a picture of someone to put in the picture frame he made; he's filled it with a picture of a woman from a magazine. He does not get to do anything that makes a human life worth living! So he turns into a bug.
In the end, Gregor does die, and his body is found by the cleaning woman. An interesting thesis might address the title of the text and whether it addresses Gregor's physical metamorphosis or the mental and emotional metamorphosis that preceded it. Indeed, his physical transformation may all be inside his own head!