Gregor the Cockroach

I think the question of why Gregor was transformed into a cockroach specifically will probably never be answered in full.  At the same time, it seems likely that Kafka will only open up even more possible interpretations.  Obviously the novel couldn't have taken the same tone had Gregor become a big cuddly cat or dog, but of all the possible creatures, why a cockroach?
Other insects such as bees and butterflies are valued by humanity at least to some extent as pollinators or honey producers in the case of bees.  Cockroaches however produce nothing of use. They are pests.  Though Gregor provides quite substantially for his family as a human, he is similarly unappreciated by his family (specifically his father) and the higher-ups at the sales firm.  This situation could only get worse for him as a cockroach, as he actually does become useless.  Gregor seems relatively unperturbed about the logistics of life as a giant insect, but becoming irrelevant and completely dependent on his family is truly his worst nightmare.  He watches his father begin to transform as well, from a feeble and lazy old man to a strong and healthy worker, essentially stepping right up to replace Gregor.  Meanwhile, Gregor is more and more overtaken by "the cockroach".

Related to this are Kafka's reasons for writing The Metamorphosis, which some psychoanalysts have decided is autobiographical.  Kafka's father was a successful businessman who pressured him to be successful.  He was strict and domineering, and similarly to Gregor, Kafka obeyed, trying to meet the expectations of those around him. As Mr. Mitchell mentioned in class, Kafka didn't leave his home very often, and wrote feverishly all night.  After Gregor's transformation, both he and the author are isolated.  The cockroach can therefore also be interpreted as a mental and communicative barrier which Kafka felt prevented him from interacting with others.

Emotional Avoidance in Paris

I think a theme in this novel that reaches further and deeper than masculinity or post-war trauma is emotional avoidance.  Every character in The Sun Also Rises but Cohn hides and/or "protects" their emotions, especially when provoked or prodded.  The subculture that these characters belong to makes an attempt to separate the public and private spheres entirely, with any infringement meeting disgust if not outright resentment.  Yet the ironic jabs at each other's personal life, company, and beliefs are commonplace and vital to conversation.  Early in the novel, Jake and Brett seem to be poking fun when they question each other's dates (a prostitute and gay men respectively), but are they really?  We read an extended passage where Jake expresses his great anger at seeing Brett with these men, but by falling into this comfortable, supposedly ironic conversation, he avoids any conflict.  Bill attacks Cohn pretty relentlessly at times, but somehow makes Cohn look out of line and out of place when he gets almost violently angry.  Even though we know Bill strongly dislikes Cohn, their subculture of emotional avoidance lets him get away scot-free.
Although I know little of Hemingway's life, he may be making a statement about living a life drowned in excess and indulgence, namely that doing so only detaches oneself from true peace and emotional and mental acceptance.  Reading the novel in this light, Jake is made an even more tragic character; Brett is able to come to terms with or at least acknowledge her insecurities and weaknesses, Cohn escapes the situation entirely, but Jake makes no progress.  Even if his time spent in solitude has allowed him to come to a stable mindset, Jake is only made more emotionally distant and passive.  With no future, Jake is merely going through the motions of life.