LORELAI: So here it is, right in front of you. Just take it. Take the plunge. She [Rachel] could be ready. Just jump in and believe her. Unless, you know, there’s some other reason you don’t want to [commit].
LUKE: Like what?
LORELAI: Like I … I don’t know.
LUKE: There’s no other reason.
LORELAI: Okay, well, fine. Then there’s no other reason.
Lorelai is giving Luke an opportunity to tell her, in private and on his own turf, that the reason he doesn’t want to commit to Rachel is because he has feelings for Lorelai. He fails to take this fairly generous hint, so they are left as friends.
It is unclear whether Lorelai is pushing Luke to commit to Rachel so he can recognise his true feelings for her, or because she wants him safely in a relationship so she can stop thinking about him, or whether, as his friend, she truly wants him to be happy. Maybe all three, although she thinks it’s the last one.