Why has Gregor changed in Franz Kafka's The Metamorphosis?

There has
never been any firm evidence as to why Gregor becomes a giant insect in Kafka's
, 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:

Whats 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...