You have an aunt who whenever she poured anything for you she would say "Say when!" 

 aunt would say "Say when!" and of course, we never did. 

We don't say when because there's something about the possibility, of more. More tequila, more love, more anything. 

More is better. 

There's something to be said about a glass half full. About knowing when to say when.

 I think it's a floating line. A barometer of need and desire. It's entirely up to the individual. And depends on what's being poured. Sometimes all we want is a taste. 

Other times there's no such thing as enough, the glass is bottomless. 

And all we want, is more.