In the first lines of The Merchant of Venice, Antonio's sadness may be attributed to a combination of several factors. He may be feeling a sense of emptiness due to his lack of meaningful purpose in life, or he could be feeling isolated from the people around him and not having anyone to confide in. He may also feel regretful for his past actions or uncertain about his future prospects. Ultimately, it is impossible to determine the exact cause of Antonio's sadness without more context.