Quote 4
PORTIA
I never did repent for doing good,
Nor shall not now; for in companions
That do converse and waste the time together,
Whose souls do bear an equal yoke of love,
There must be needs a like proportion
Of lineaments, of manners, and of spirit;
Which makes me think that this Antonio,
Being the bosom lover of my lord,
Must needs be like my lord. (3.4.10-17)
Portia recognizes the depth of the friendship between Antonio and Bassanio. Rather than resent it, she inserts herself into the situation. She won't fight Antonio; she'll actually help him. In the process, Antonio will go from being a competitor to being in her debt. What's more, Portia will come off as benevolent and magnanimous.
Quote 5
PORTIA
You must take your chance,
And either not attempt to choose at all
Or swear before you choose, if you choose wrong
Never to speak to lady afterward
In way of marriage. Therefore be advised. (2.1.40-44)
It seems rather arbitrary and harsh that the condition to try for Portia's hand is that losers can never seek marriage... ever again. Still, it holds up marriage as a really serious affair and helps separate the men from the boys in terms of who's really willing to sacrifice for the chance to marry Portia.
Quote 6
PORTIA
Happiest of all is that her gentle spirit
Commits itself to yours to be directed
As from her lord, her governor, her king.
Myself, and what is mine, to you and yours
Is now converted. But now I was the lord
Of this fair mansion, master of my servants,
Queen o'er myself; and even now, but now,
This house, these servants, and this same myself
Are yours, my lord's. I give them with this ring,
[handing him a ring.]
Which when you part from, lose, or give away,
Let it presage the ruin of your love,
And be my vantage to exclaim on you. (3.2.167-178)
According to Portia, marriage is about a woman giving over herself (and all her money and property) to a partner. This is what she does here when she gives Bassanio "this house, these servants" and a ring. (In the 16th century, when a man married a woman, she automatically became his property and legal responsibility.)
At the same time, Portia is also being pretty crafty here. Because she gives him more than he can possibly give her in return, Portia binds Bassanio to her. Check out what we have to say about Portia's ring in "Symbolism, Imagery, Allegory" for more on this.