Early in The Metamorphosis, we learn that Gregor dearly wishes to quit his job and be free of his family obligations. Being turned into a bug handily takes care of this problem for Gregor – you could say it's overkill. Gregor's physical isolation from the outside world in his room speaks to his general alienation from modern society, which expects him to work hard and find a wife. Despite the fact that he's finally gotten his wish, Gregor is overwhelmed with feelings of guilt and shame at being – literally – a parasite to his family. Gregor's retreat from human society causes him just as much unhappiness, if not more, than when he was a functioning member of society. In the bleak world of the novella, happiness is impossible because the needs of the individual and society are irreconcilable yet equally compelling.
Gregor's life as a vermin is a metaphor for feelings of alienation and isolation that existed long before his transformation.
The devolution of Gregor's room from a human bedroom to a storage closet reflects how his connection to human society deteriorates as the story progresses.