why do you think Antonio is sad in the begging?

The depression of Antonio at the beginning, for which he can give no explanation, is much like Antipholus of Syracuse in The Comedy of Errors. Portia, the wealthy Belmont heiress, is likewise a depressed and unhappy character in the opening scenes. The reasons for their melancholy, although never directly expressed, are due to their self-absorption. And as with Antipholus in The Comedy of Errors, it is only by taking a huge risk (or both) that they will be able to overcome their depression. For Portia, this risk taking can be seen in her love for Bassanio, which will require her to risk her entire inheritance in order for her to win him. For Antonio, the risk is even greater; namely a pound of flesh, representing his very life.