The answer to this question can be found at the beginning of the second part of this perplexing story, when Gregor goes down onto the floor of his room to discover that his sister had left for him a bowl full of milk and bread, which used to be his favourite drink. However, in spite of his initial enthusiasm, he quickly realises that his new condition has changed more than just his physical appearance:
Soon, however, he withdrew his head in disappointment. Not only did the bruises on his left side make it difficult for him to eat--he could eat only if his entire wheezing body joined in--but he did not care for the milk, even though it had always been his favourite...
The injured state that Gregor finds himself in means that it is now difficult for him to drink the milk, but also he finds that his physical transformation has also begun to transform other aspects of his character as well, in terms of his tastes and likings.