LUKE: But why, what happened?
LORELAI: Jess happened.
Lorelai completely blames Jess for Rory’s break-up with Dean. Although in a sense she’s correct, in that Jess’ appearance precipitated the break-up, Lorelai ignores the fact that her daughter hadn’t been happy with Dean for a long time. Rory and Dean didn’t share many interests or similar values, and it seemed as if when Rory wasn’t frightened of Dean’s temper, she was bored with him. Lorelai was happy for Rory to continue in an unsatisfying relationship rather than go out with Jess.
Unlike Lorelai, Luke is thrilled by this news. He has been shipping Rory and Jess since the beginning, and this is another instance where Luke and Lorelai are not on the same page.