When the play opens, we are introduced to an Antonio who is depressed, but he does not know why. He informs Salanio and Salarino:
"In sooth, I know not why I am so sad."
It becomes immediately obvious that Antonio might be prone to inexplicable mood swings. He at times experiences certain emotions for which he has no explanation.
We learn through Salanio and Salarino that Antonio is a merchant and the two try to help him identify the reason for his ill-mood, suggesting that he might be concerned about the safety of his cargo at sea. He might fear their destruction and the resultant loss of income.
It is clear that Antonio is an astute businessman for he assures the two that:
"Believe me, no: I thank my fortune for it,
My ventures are not in one bottom trusted,
Nor to one place; nor is my whole estate
Upon the fortune of this present year:
Therefore my merchandise makes me not sad."
When Salarino suggests that he might be in love, Antonio fervently rejects this notion as well.
In this discourse we learn that Antonio has many friends and he holds them all dear. He affirms his affinity by telling Salarino:
"Your worth is very dear in my regard"
It is clear that he is much respected and admired, a fact confirmed with the arrival of Bassanio, Gratiano and Lorenzo. Gratiano immediately notices that Antonio is not well and says:
'You look not well, Signior Antonio;
You have too much respect upon the world:"
Gratiano indirectly suggests that Antonio has a quiet wisdom about him and that he has a calm and steady demeanour. Bassanio, however, later mocks Gratiano, stating that he speaks too much but says very little.
We discover in his conversation that Antonio is also extremely generous, and unstintingly loyal, specifically toward Bassanio. We learn that Bassanio needs money and that he already owes Antonio a good deal, but Antonio unflinchingly promises to help him, saying:
"My purse, my person, my extremest means,
Lie all unlock'd to your occasions."
Antonio is even prepared to use his credit-worthiness in Venice to raise a loan in order to help Bassanio. A true friend indeed!
It is clear that Antonio and Bassanio share a very deep affiliation. Many commentators have suggested that they might be in a relationship themselves, however, that is a matter of interpretation. What is clear is that Antonio does have a deep liking for Bassanio and would do practically anything to help his friend, and, as we discover later, even risking his own life.