Dean pissed me off here. He still seems bitter about the fact he and Rory grew apart. In his mind, he feels like he was left behind multiple times by Rory and still holds residual anger there. He had his hand in what wrong in their relationship as well, though I don't think he was in a place to see that just then. He was just projecting his own issues and bitterness on to Luke. Granted, Luke was kind of a dick to Dean most of the time because of Luke's "papa bear" instincts for Rory. It just wasn't necessary of Dean to try and throw a monkey wrench in Luke's relationship with Lorelai.