It was the dress of queens in fairy tales, of a red deeper than blood. […] Herself meeting herself. (1.66)
In this odd scene from the beginning of the book, Therese tries on a red dress and feels like she's seeing her true identity. Nothing like this happens again, but red is a traditional literary symbol of lust and passion, so Therese is seeing an inner passion on the outside.
It was the hopelessness that terrified her and nothing else. […] The hopelessness of herself, of ever being the person she wanted to be and of doing the things that person would do. (1.75)
Therese sees her inner passion when she tries on the dress, but she's unsure what it is. At this point, she can't be the person she wants to be because she's unsure of who that is.
"My name? Carol. Please don't ever call me Carole." […]
"How do you like it pronounced? Therese?"
"Yes. The way you do," she answered. Carol pronounced her name the French way, Terez. (4.45, 4.47-4.48)
This is a weird little exchange, but it shows that Therese and Carol get each other on a certain level. But how does anyone else pronounce Therese's name? She never says.
"The hardest thing to be." (9.80-9.81)
Carol and Richard are the proto-hipsters hipsters of the 1950s, aspiring to be nothing because somehow having no identity counts as having an identity.
"I should think you'd be good for her. Because Carol's so serious." (10.53)
When Abby says this, we understand that Carol isn't a little frosty only toward Therese—she's this way with everyone. It's part of her stone-faced personality.
She complemented Carol's solemnity, she could remind Carol to laugh. (10.41)
Perhaps opposite identities attract, but Therese has to become more than just Carol's amusement before the two of them can have an equal relationship.
Therese hesitated uncomfortably. "I just don't feel in the mood for sitting these days." (12.23)
Therese is talking about sitting while Richard paints her, but her words have a double meaning: Therese is starting to become restless. She understands that in order to find herself, she can't sit still, and she can't let Richard define her either.
"You don't know yourself." (13.37)
This is Therese speaking to Richard after he basically says the same thing to her. They're around the same age, late teens to early twenties. Does anyone really know who they are at that age?
"I HAVE NOT CHANGED. NEITHER HAVE YOU. WRITE TO ME. I LOVE YOU. RICHARD." (14.13)
Richard goes back and forth with saying that Therese has changed, and other people later comment that Therese has changed, too. This is a good thing for her, because Therese wants to change. She wants to mature. Richard, by contrast, wants to keep everything the same.
"All right. Come on. Let's get to Des Moines. How about you driving a while?"
They changed places. (19.174-19.175)
The short three-word sentence at the end here is another line with a double meaning. In the plot, the two ladies are simply switching places in the car. But this is the beginning of the role reversal, where Therese starts to be the one in charge of the relationship and Carol becomes her passenger.