Jealously

"I was blind, unforgivingly jealous of what had happened to [Robert]" (page 105).

Everything about Brett is becoming a little bit clearer as I read through the book. Robert had one date with Brett, and fell head over heels for her. I don't think Brett sees Robert the same way he does her. Clearly though, most men who meet Brett feel the same way. Now Jacob, I think, had a past with Brett, but not a romantic past. I mean, I think they've been.. well, they've done it. I'm pretty sure. (Because of the whole kissing scene in the taxi a while back). Brett and Jake's relationship has never been exactly what Jake has wanted... they have never been serious. And because Robert had an actual date with Brett, he rubs it in Jake's face. This makes Jacob extremely jealous and passive aggressive (torturing Cohn with the telegram).
But in reality (I hate that phrase), I think Robert is only into Brett for her looks and spontaneity-he just wants to live a little, and Jake is actually interested in Brett's personality. He loves her. Robert knows that and he is being a real jerk.