What are the reasons suggested for Antonio's sadness ?

Antonio in unsure about his sadness. Salarino suggests that his mood is caused by his worry over his trade ships at sea. He also suggests that possibly.... Antonio is in love. When Antonio denies this, Salarino says;

Then let us say 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.

Translation.... the only reason for Antonio's sadness is the fact that he isn't happy.