David does not quite know how it happened that he and Rosalind fell in love. The events of the story span years, and David and Rosalind have grown up together because they are cousins. Their families feud with each other, so it was entirely possible that David and Rosalind would grow up hating each other, but the feud had a way of solidifying them in a sort of unity with each other. Their telepathic ability also gave them a huge common bond with each other. The bond is so important that Rosalind refuses to act upon the overtures of other men. She simply feels more connected with David, and David reciprocates those feelings.

She would not be entangled with any of them. Very likely it was for that reason that she was more shocked than any of us by what Anne proposed to do.

[. . .]

Marriage to a norm, even the kindest and best of them, was unthinkable for both of us.

To be a bit of a cynic about the relationship, I think David and Rosalind fell in love with each other because there simply weren't too many other options. Anne even calls them out on this. She uses it to defend her falling in love with a norm. The story is written from David's perspective, so readers are quite limited in knowing Rosalind's feelings, but David makes it clear that teenage hormones are partly to blame for driving him toward Rosalind. He makes it clear that she is growing up into a beautiful woman, and that is why other men are making passes at her. Readers also know that David does like/love the telepathic connection he has with her, but there are several paragraphs where David says he loves a bunch of things about her; however, he only lists her physical traits.

I loved the girl one could see. I loved her tall slim shape, the poise of her neck, her small, pointed breasts, her long, slim legs: and the way she moved, and the sureness of her hands, and her lips when she smiled. I loved the bronze-gold hair that felt like heavy silk in one's hand, her satin-skinned shoulders, her velvet cheeks: and the warmth of her body, and the scent of her breath.

The love story between David and Rosalind is a sweet one in The Chrysalids. As telepaths, the two have shared every thought since they were young among themselves and the other mutants in the community. David says he has loved her as long as he can remember, and their shared thoughts have made them incredibly close and intimate.

David knows from a young age that the two will get married, and the other telepaths recognize their love. They are pointed to as pillars of love for the telepath community, as Anne recognizes when she falls in love with a non-telepath named Allen.

David and Rosalind have to sneak around for the majority of the story due to their parents' disagreement. However, when the telepaths finally flee the community and make a way on their own, the two are able to be together.

David says that he has loved Rosalind as long as he can remember.

All of the telepaths are very close.  Being able to read another person’s mind is an intimate thing.  David and Rosalind grow to love each other while they are young, and as they get older they realize that they will marry one another someday. It just seems natural.

Quite when it was that we had known we were going to marry one another, neither of us has been able to remember. It was one of those things that seem ordained, in such proper accord with the law of nature and our own desires, that we felt we had always known it. (Ch. 10) 

When Anne falls in love with a man who is not a telepath, Allen, she tries to explain to the others that they will not understand because none of them have been in love except David and Rosalind.  The others tell her that she can’t marry a normal person, but as Uncle Axel tells David, you can’t question a woman in love.

David and Rosalind’s relationship is forbidden because of a feud between their parents.  It does not stop them.  Their love and their unusual abilities make them above such considerations.  Still, they have to meet in secret.

We used to meet, discreetly and not dangerously often. No one but the others, I think, ever suspected anything between us. We had to make love in a snatched, unhappy way when we did meet, wondering miserably whether there would ever be a time when we should not have to hide ourselves. (Ch. 10) 

Because of their secret, David wonders what would happen if Rosalind got pregnant.  His parents would not approve of the marriage, and his father is powerful.  Eventually, the telepaths will have to go on the run. 

Everything from Anne blows up, and their cover is blown.  Alan is killed and Anne thinks one of the telepaths did it and commits suicide.  Soon after, Petra panics when her horse is being attacked by a mutant and calls the others.  They have a hard time explaining how they knew to go there since she made no sound, and their secret is out.