Where and why in the play The Merchant of Venice is it shown that Antonio belongs to the character type of a melancholy man?

Antonio's melancholy is the topic of conversation at the
opening of the play, Act I, scene i.  He begins with these words: "In sooth, I know not
why I am so sad,"  and though his comrades try to attribute his emotional state to the
fortunes of his ships full of merchandise on an uncertain sea or love, Antonio
repudiates both, remaining puzzled by his own
emotions:

It
wearies me.  You say it wearies you.

But how I caught it,
found it, or came by it,

What stuff 'tis made
of...

I am to learn.

And such
a want-wit sadness makes of me,

That I have much ado to
know myself.

Antonio
confesses that he really doesn't have any idea why he's sad, and it seems that he's been
in this funk for awhile, since he mentions that both he and his comrades are "weary" of
it.

Finally, one of his comrades concludes that he is
"sad/Because [he is] not merry."  This sadness without cause is often referred to as
melancholy, and Antonio accepts his role in the
world:

I hold
the world but as the world, Gratiano,

A stage where every
man must play a part,

And mine a sad
one.