Men like [Mark]—men who are obviously interested purely in what they think of other people, not in what other people think of them—have always made me violently insecure. (2.40)
Violently insecure, yes, and jealous, too. Jealous of how Cassie doesn't seem to hate Mark and is the object of her attention… even though she's only paying attention to him because Mark is the subject of a murder interrogation.
"Please, Ryan, do us both a favor and grow up; you know exactly what I mean. [Rosalind] smiles at inappropriate times, and, as you spotted, she wasn't wearing a bra." (4.57)
Cassie doesn't say these things because she's jealous of Rosalind (unlike Rob who tries to pin the blame on Mark, whom he's jealous of). But Rob acts as though Cassie is jealous of Rosalind, because he (Rob) is the type to be jealous over something so silly.
I checked the interview room: Mark was writing, and Cassie had managed to make him laugh. […] Apparently they were managing to get by without me. (6.134)
"Apparently." That one word makes us think that Rob is furious that Cassie is making Mark laugh. Why isn't she making Rob laugh, huh? What about him?
"[Mark's] motive is he's several hundred yards out of his tree. […] When it comes to this site, he's batty." (8.78)
We don't have the space, or the patience, to write Rob's whole rant on why Mark is guilty. It's almost entirely speculation derived from the jealousy he feels toward the man.
"Shane found out and wanted to play, too. Cathal was of course fine with this, but Sandra wasn't." (12.61)
Shane, Jonathan, and Cathal share everything. Note we said "thing." The problem is that they see women as things, so Shane expects Cathal to share Sandra, too—he's jealous that she sleeps with Jonathan and Cathal, but not him.
"She's never liked me very much, has she?" (17.53)
Rosalind pretends to be jealous of Cassie to gain sympathy from Rob, and to subtly turn Rob against his partner, especially when Cassie suggests that Rosalind might be a part of the murder.
Sam had spent much of his life trundling happily along on sheer dumb luck, and I had a hard time working up much sympathy for the fact that he had finally stepped on a banana skin and gone flying. (18.41)
We have to wonder if Rob is jealous of Sam's innocence. Innocence isn't something Rob really ever had since almost getting murdered at twelve years old takes that away pretty quickly.
It didn't help that Peter and Jamie looked exactly like they always had: longer in the leg, all their baby teeth gone, but still slight and light and invincible as ever. (18.130)
Rob seems to be a little jealous here, that as he got older, he entered an awkward phase that his friends seem to have avoided.
"I'm aware of that. I can." (20.8)
Dude. Rob. Let it go and stop trying to pin this whole crime on Mark just because you don't like him and Cassie does. Heck, she doesn't even like him. She just doesn't think he killed and raped a twelve-year-old. There's a big difference.
"You've never liked Rosalind, have you?" (21.99)
And In the Woods practically turns into an episode of Pretty Little Liars here as Rob accuses Cassie (a grown woman) of not liking a teenager. Rob is the type of person to let his jealousy control him during a police investigation (see all the quotes about Mark), so he acts like Cassie is the same way, but she is much more professional.