In Act 1, Scene 1, Antonio confesses his sadness to two of his friends, Salerio and Solanio. He adds that he has no idea "how I caught it, found it, or came by it,/What stuff 'tis made of, [or] whereof it is born."
Salerio and Solanio advance three guesses about the cause of Antonio's sadness. None of their guesses rings quite true.
Their first guess is that Antonio is worried about the fate of his ships, into which he has sunk a lot of money, and which are all out on trade missions to various parts of the world. Salerio goes into some detail about how, if he were in Antonio's position, every little thing would remind him of his ships. He adds a vivid word picture of "dangerous rocks,/Which touching but my gentle vessel's side/Would scatter all her spices on the stream."
Antonio denies that he is worried about his ships. Not all his wealth is sunk into one fleet, he says, nor is his whole "estate" depending on the outcome of this year's venture.
Solanio then guesses that Antonio is in love. Antonio dismisses this idea with a "Fie, fie!"
Solanio then advances the third guess, which is that
you are sad/because you are not merry; and 'twere as easy/For you to laugh and leap, and say you are merry/Because you are not sad.
In other words, Solanio describes a creeping malaise or boredom that is not exactly sadness, just a lack of zest or enjoyment in life (very similar to some manifestations of depression). Antonio does not get a chance to confirm or deny this, because some other characters enter at this point.
Though none of the three explanations seem to fit perfectly, the one about the ships does several things. It gives us critical background information about Antonio's ships. It shows us Antonio's overconfident attitude about his money, which will get him into trouble later. And it suggests a possible fourth explanation for Antonio's sadness: Though he is not consciously worried about the ships, perhaps the fact that they will all sink later, is causing him some foreboding now. This would then be a case of "future events cast backward shadows."