| Quote #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 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 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.