"Pray, sir," cried Vivaldo, "oblige us with an account of her parentage, and the place of her birth, to complete the description." (18.104.22.168)
When Don Quixote starts telling the world about the beauty of his beloved Dulcinea, the first thing everyone wants to know is where she was born and who her parents are. Because seriously, this information would be more important to someone from 17th-century Spain than any information about whether Dulcinea were a nice or beautiful person.
"Customs did not come up all at once, but rather started up and were improved by degrees; so thou mayest be the first earl that rode in state with his barber behind him." (22.214.171.124)
In a private moment, Don Quixote tells Sancho that if he (Sancho) becomes an earl, he'll be free to make up whatever customs he wants, like having his barber travel around with him. After all, once you're rich and powerful, people will start doing whatever you do.
"[For], as I said, he was a very fine comely young man, and by his speech and behavior we could guess him to be well-born, and a court-like sort of a body." (126.96.36.199)
The shepherd says that he realized from Cardenio's appearance that Cardenio came from a fine family. He knows this in spite of the fact that Cardenio has been running around attacking shepherds at random. None of this really seems to matter because Cardenio comes from a noble, wealthy family. That in itself makes him respectable.
"My name is Cardenio, the place of my birth one of the best cities in Andalusia; my descent noble, my parents wealthy." (188.8.131.52)
As Cardenio introduces himself, he gives the most important information first. He comes from a good city and from noble, wealthy parents. People would no doubt recognize his family's last name if he'd bother to mention it. But what matters is that Cardenio comes from a long line of wealthy people, which automatically makes people respect him.
Our observers were amazed at the discovery, rightly imagining that such tender feet were not used to trudge in rugged ways, or measure the steps of oxen at the plough, the common employments of people in such apparel. (184.108.40.206)
When they first see the beautiful Dorotea washing her feet in a mountain spring, the men figure that there's no way that someone with such delicate feet could come from a working-class background. Whoever this is, she clearly hasn't ever plowed a field. And that's because Dorotea has spent her whole life shut up like a nun by her father, who is a wealthy man.
"I am not the first, thought I to myself, whom marriage has raised to unhoped-for greatness, and whose beauty alone has supplied her want of birth and merit." (220.127.116.11)
Dorotea tries to explain how her beauty is strong enough to make her think that she could marry a noble lord. After all, she realizes that class means a lot to people in Spain, but every now and then, a woman's sheer beauty can make a man reach beneath his class for the sake of marrying.
"[But] a young lord, heir to a great estate, and has such a full possession of my heart, that if he does not slight it, it must be his for ever." (18.104.22.168)
For the first time, Dorotea mentions the man who has broken her heart by rejecting her. This man is none other than Don Fernando, the same guy who betrayed Cardenio. But for all of his poor treatment of Dorotea, Dorotea is still devoted to him, and she no doubt makes excuses for him because of his high social class.
"If young girls might always choose their own husbands, we should have the best families intermarry with coachmen and grooms; and young heiresses would throw themselves away upon the first young fellows, whose promising outsides and assurance make them set up for fortunes, though all their stock consists in impudence." (22.214.171.124)
Don Quixote is willing to admit that sometimes people should choose their own spouses. But he doesn't want this to become a widespread thing, or else the upper classes and the lower classes will start mixing too much, which will leave the country with no more well-bred people, just a bunch of mutts. It's interesting, though, that what Don Quixote is worried about isn't so much interbreeding in itself but the lack of education and refinement he thinks this will cause.
"[If] my government happens to last four days to an end, it shall go hard but I will clear the island of those swarms of Dons that must needs be as troublesome as so many flesh-flies." (126.96.36.199)
Sancho Panza has no time for people running around with fancy titles like "Don" in front of their names. He says that if it were up to him, he'd do away with these kinds of titles and make it so that everyone was treated equally. To people from 17th-century Spain, though, this sounds like too radical of an idea.
"The case then is, noble sir, that, though you see me sitting in this chair, in the middle of Arragon, in the habit of an insignificant, unhappy duenna, I am of Asturias de Oviedo, and one of the best families in that province" (188.8.131.52).
When Doña Rodriguez sits next to Don Quixote's bed, she opens her speech by telling him that while she might work as a servant, she comes from a very noble family. Her ancestry is supposed to tell Don Quixote that she is descended from a noble family, regardless of what her current occupation might suggest. In this case and many others, people of noble descent always make sure that this is the first thing they mention about themselves. Here we see, also, that the relationship between nobility and money is complicated. Money in and of itself isn't enough to make a person noble or respected, but a nobleperson without money, like Doña Rodriguez, is still forced into servitude. Being a duenna is nothing like being a lowly rank-and-file servant, but it's still not that great.