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!

Kafka leaves it to the reader to decide why Gregor Samsa wakes up one morning as a giant insect. From the hints given in the story, however, it seems clear that Gregor felt dehumanized by his unfulfilling life. When he lies in bed in the morning, before he realizes he has changed, he complains bitterly to himself about his job as a traveling salesman, which he hates. He wants nothing more than to be able to quit it, but is doing it to repay his parents' debts. His family seems disinterested in him as a human being, focused instead on his capacity to make money. He does die.

We can understand how all of this loathing of his life and self could have built up in Gregor to the point he transformed literally into what he felt like on the inside.

This transformation into an insect releases Gregor from the job he hates and also reveals the indifference his family has towards him once he can no longer bring home earnings.

A thesis might focus on whether Gregor could have done more to escape his fate. Did he really have to work at a job he hated to settle his parents' debts, or was he socially conditioned into taking on responsibilities that were too much for him? He probably was socially conditioned, but you might explore if he could have escaped this by being less passive about his life.

There is no reason given for Gregor Samsa's transformation. The opening lines are as much explanation as is ever given; the characters in the book show less surprise at the actual event and more disgust that Gregor is a giant insect. The metamorphosis itself is simply a given; there is no justification, or any reason that Gregor's human mind remains intact. Even Gregor himself is not so worried about the change:

"This getting up early," he thought, "makes one completely idiotic. A man must have his sleep. Other travelers live like harem women."

One implication is that his transformation is simply another joke played by an uncaring universe. Gregor is unhappy at home and at his job; despite his optimism that his hard work will change his status, he believes in determinism, and thinks that he will never truly escape from his position. In that case, his physical change is simply a representation of his mental state; he thinks of himself as a drone of the state and of his job, and so he becomes an insect, which operates in a hive-mind and has no autonomy of its own.

Another possibility is that he is transformed because the plot demands it. In other words, Gregor is a pawn of the author as much as of anyone else; he changes physically because otherwise, there would be no story. Here, the needs of narrative intrude on Gregor's reality and alter him as necessary.

Kafka introduces the story as Gregor wakes up as a bug.  One of the best aspects of the story is that the narrator never does directly say "and this is why Gregor became an insect," leaving the reader to draw inferences from their own perspective.  Certainly, Gregor's unhappiness with his current situation raises some red flags; his job stresses him out. 

“Oh God,” he thought, “what a strenuous occupation I've chosen! Always on the road, day out, day in. The rigors of the job are much greater than if I were working locally, and, furthermore, the nuisances of traveling are always imposed upon me...

He has not done very well for himself as a salesman, and his in-debt family seems very dependent on him.  Gregor's unfortunate transformation does have the positive side-effect of relieving him of responsibility.  He probably wished he could be free from his burdens; well, now he can.  As a bug, all he has to do is crawl around all day, and in turn, his family now feels the pressure to take care of themselves. 

Gregor is a travelling salesman, and not a happy one.

AS GREGOR SAMSA awoke one morning out of restless dreams, he found himself in bed, transformed into a gargantuan pest. (ch 1, p. 4)

When Gregor is transformed, it forces him to re-evaluate his life.  It is what we would call a "wake-up call" or perhaps a mid-life crisis.  Instead of buying a red convertible, Gregor becomes a giant bug.  Either way, he needs to stop and think about his life and where he is, and take a personal inventory.

Turning Gregor into a bug also forces the reader to stop and think about life differently, and maybe think about his or her own life. I think you could argue that this was definitely Kafka's goal.