* His business is failing: Antonio says he has "lost a ship" and is worried about the rest of his fleet. The play's setting during the Renaissance was a time of immense trade and risk, and losing a ship could be a significant financial blow.
* He's in love with Bassanio: Antonio's melancholy is linked to his deep love for Bassanio, who is about to embark on a journey to win Portia's hand in marriage. Antonio's unspoken fear is that Bassanio may leave him, or that his love for Bassanio is unrequited.
* He's a romantic soul: Antonio, as a character, is often described as deeply emotional and sensitive. He is prone to melancholy and introspective thought, which may contribute to his overall sadness.
* He's possibly experiencing unrequited love: Some scholars have argued that Antonio's melancholy is a sign of unrequited love, possibly for Bassanio. However, this interpretation is debated and not definitively stated within the play.
Overall, Antonio's unhappiness is a combination of personal and financial worries, which are further deepened by his romantic nature and possibly unrequited love. This melancholic mood sets the stage for the play's central conflict, where Antonio's willingness to risk everything for his friend sets him on a path of danger and hardship.