The feeling bore no resemblance to what she had read about love. Love was supposed to be a kind of blissful insanity. (2.67)
There are two ways to learn about love—to read about it and to feel it. Therese has only read about it, but are books an accurate way to depict love? Or do they make it more dramatic than it is? What about this book? Is it accurate or dramatic?
It would be almost like love, what she felt for Carol, except that Carol was a woman. It was not quite insanity, but it was certainly blissful. (5.14)
Therese recognizes that her feelings for Carol are atypical, or at least unlike the male-female relationships she's read about in books. But she doesn't let this scare her away or gross her out. She not only accepts her feelings, she pursues them.
Yet the simple fact that she wasn't in love with [Richard] made Therese feel guilty, guilty about accepting anything from him. (5.54)
Therese feels guilty, but she isn't leading Richard on. She lets him know flat out that she doesn't love him, but he says he loves her, so he sticks with her.
"I don't love you, but I like you." (5.60)
This is Therese to Richard, and it seems like Richard is the opposite—he loves Therese but doesn't actually like her, which makes us wonder if he understands what love is. If he liked Therese as a friend, he might be a little more understanding about what she's going through.
Was it love or wasn't it that she felt for Carol? And how absurd it was that she didn't even know. (8.77)
How does a person know when they're in love? It's a difficult emotion to define, which is why entire books are written about it. The only thing Therese can know for sure is that Carol makes her feel really good.
"Do people always fall in love with things they can't have?" (9.41)
A forbidden aspect almost always enhances the feelings of love. See: Romeo and Juliet. Carol and Therese also have a forbidden-love aspect to their relationship because being a lesbian isn't socially acceptable.
"What real reason have you to think he's not in love with you?" Carol asked. (13.70)
As someone who hasn't been in love, Therese isn't qualified to judge whether or not Richard is in love with her. But the way he acts as their relationship falls apart makes us think that he doesn't love her.
What was it to love someone, what was love exactly, and why did it end or not end? Those were the real questions, and who could answer them. (16.61)
We present this quote with no comment. Are there answers to these questions? Are you the one who can answer them? There is no question mark at the end of that line, perhaps because Therese believes there is no answer.
"After that, I knew I was in love with Abby. I don't know why not call it love, it had all the earmarks. But it lasted only two months, like a disease that came and went." (16.52)
Would Carol call love a "disease" if she experienced the same feelings with a man?
How was it possible to be afraid and in love, Therese thought. The two things did not go together. (18.7)
This is a good point, and one reason why Therese is in love with Carol but believes Richard doesn't love her. Therese feels safe with Carol, whereas with Richard she feels afraid and insecure.