In you read the beginning of Franz Kafka's The Metamorphosis closely, you will find that while his feelings of alienation and isolation have been building for a long while, Gregor's transformation is sudden.
The narrator notes that Gregor wakes from troubling dreams to find himself altered:
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.
There is nothing to specifically blame his transformation on, but Gregor considers that it may well be his job:
“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...”
However, as the story unfolds, the concerns related to his erratic schedule and long hours of travel fade to find something more serious at the heart of his "isolation." While Gregor has spent years working hard so that no one else in his family needs to find employment, what was originally welcomed by his parents and sibling is now taken for granted.
...[Gregor] 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...
As it becomes glaringly obvious that their "meal ticket" (Gregor) shows no signs of being able to support them further, the reader notes two important things: that Gregor's father had money put aside (from his failed business) that he never told Gregor about—even though having this money might have lightened his work load—though Gregor can only rejoice that the family is not financially lost.
Now, Gregor found out clearly enough...that, in spite all bad luck, an amount of money, although a very small one, was still available from the old times and that the interest, which had not been touched, had in the intervening time allowed it to increase a little. Furthermore, in addition to this, the money which Gregor had brought home every month—he had kept only a few crowns for himself—had not been completely spent and had grown into a small capital amount.
Second, Gregor's father and sister begin to turn on him because he no longer has value as the money-, indicating that he also has no value as a son or brother.
While it is uncertain if Gregor is dreaming during the story, if he feels symbolically like a giant, disgusting insect, or actually turns into a bug, his family eventually stops caring for him at all and he simply dies—as if from a broken heart.