In Franz Kafka's The Metamorphosis, how did Gregor become a traveling salesman?

In Franz Kafka's href="http://en.wikipedia.org/wiki/Novella">novella, The
Metamorphosis, (published in 1915), Gregor Samsa awakes one morning as a
"monstrous insect." His concern is not so much that he has turned into an enormous bug,
but about how difficult his job is and what a toll it has taken on him. It is almost as
if he sees his altered state as a result of:

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...the stresses of selling[: there are] the
problems of travelling, the worries about train connections, irregular bad food,
temporary and constantly changing human relationships, which never come from the heart.
To hell with it all!”

There
is always debate as to whether Gregor literally turns into an
insect overnight, which would understandably surprise, and perhaps
disgust his family, or if the novel is speaking figuratively: that
once Gregor stops working—does not get out of bed that morning and get on the train—he
becomes a pariah to the family—people who do not work, but pursue their personal agendas
while Gregor works desperately hard to pay the  family's
bills.

How does Gregor end up this way? We read later in
the story that he was in the military at one point. However, when
we meet him at the story's beginning, Gregor explains why he has
this specific job:

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Once I’ve got together the money to pay off my
parents’ debt to [the boss]—that should take another five or six years—I’ll [quit] for
sure.

Regardless of whether
this is figurative or literal—whether it is a dream (as Gregor wonders when he wakes up)
or an actual event, Gregor is unable to go to work. Though he would
willingly try to do so, as he has for so long, he simply cannot maneuver and his
condition rapidly deteriorates. It is soon apparent that whatever the case, his value to
his family rests firmly on his ability to provide a paycheck, and except for his mother,
who eventually also turns away, sadly he is worthless to them from this point
on.

Note:

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A novella has
generally fewer conflicts than novels, yet more complicated ones than short
stories.

Additional
source:

http://en.wikipedia.org/wiki/Novella