* His ships are at sea, and he is worried about their safety. Antonio has invested a large amount of money in his ships, and he is anxious about the possibility of losing them in a storm or to pirates.
* He is in debt to Shylock, a Jewish moneylender, and he is worried about being able to repay the loan. Antonio has borrowed money from Shylock to finance his shipping ventures, and the terms of the loan are very harsh. If Antonio cannot repay the loan on time, Shylock will be entitled to a pound of his flesh.
* He is in love with Portia, a wealthy heiress, but he believes that she is out of his reach. Antonio is a humble merchant, while Portia is a member of the nobility. He believes that she would never consider marrying him, and this thought makes him sad.
These are just some of the reasons why Antonio is sad at the beginning of the play. His sadness sets the tone for the rest of the play, which is full of conflict and tragedy.