Also, the writing is filled with rapid-fire pop-culture references (obscure ones, even) so when you re-watch, you'll often notice some little thing that you hadn't noticed before. And it's always delightfully funny.
This article summarizes it nicely:
Ode to Gilmore Girls: Behind the Musicality of TV's Beloved Show
And, though it's already been stated: It's comforting. Truly the ideal thing to watch if you're horribly stressed in real life on a fairly regularl basis. (..Considering that, I should probably re-watch it, too. )