Why does Lorelai just walk in Sookies house but Luke always knocks?

In one episode Lorelai is babysitting Martha and Davie and she just walks in. Understandable, i do the same with my friends, but everytime Luke is visiting Lorelai, while they're engaged, he knocks? Like why? I get respectful and all but theres talk of them living together within the year and he still knocks? He doesnt have a key? He isnt welcome in at a moments notice?? Its not like he can't walk in of on her naked? Just why?