WHAT IS ANTONIOS ACTUAL CAUSE OF SADNESS IN ACT 1 SCENE 1 ?

In Act I, Scene 1, we are told that even Antonio doesn't know why he's feeling sad. Everyone assumes he's worried about the safety of his ships, but he claims the ships are not the reason for his depression. 

There are many probable causes for Antonio's sadness .the most possible reason as speculated by both salarino and salanio is that antonio's mind is diverted by the severe threats his fortunes are exposed to, like tempests, pirates , currents, and boulders in the shallow waters.antonio denies that by stating that he has diversified his assets and also denies the assumption of his pals that he is in love. the other possible reasons might be that he has no family to share his emotions with or he is melancholy about losing bassanio to portia. However ,gratiano says that antonio likes to remain as a silent and melancholy creature to construct his reputation as a dignified personality .