How we cite our quotes: (Act.Scene.Line). Line numbers correspond to the Norton Shakespeare edition.
Quote #1
ANTONIO
In sooth, I know not why I am so sad.
It wearies me; you say it wearies you;
But how I caught it, found it, or came by it,
What stuff 'tis made of, whereof it is born,
I am to learn;
And such a want-wit sadness makes of me
That I have much ado to know myself. (1.1.1)
We don't yet know why Antonio is sad, but his mysterious proclamation sets him up as a generally melancholy character for the rest of the play. Furthermore, we learn that Antonio doesn't always have a rational explanation (at least one that he knows of) for how he feels.
Quote #2
PORTIA
If I live to be as old as Sibylla, I will die as chaste as
Diana, unless I be obtained by the manner of my father's will. (1.2.15)
Portia would prefer a life of isolation over disobeying her father's will. Still, given the guys she's looking at, it's no great loss not to marry. Portia is strong-willed and doesn't seem like a romantic; she bravely faces the possibility of dying old and alone.
Quote #3
SHYLOCK
I will buy with
you, sell with you, talk with you, walk with you, and so
following; but I will not eat with you, drink with you, nor pray
with you. (1.3.9)
Shylock is isolated from Christian society. He can engage with Christians in business dealings, and so he has a livelihood, but it's clear from this passage that he keeps his distance socially. He's a Jew in a Christian country, which explains the animosity we see from and toward him.