How do you know when a stereo sounds good?

Silly like asking what music is good ? Obdurate not obtuse but certainly agenda driven. 

when i lose track of time.

The other question I have is why do you sit down one day

and absolutely love the way your system sounds yet the

next day conclude it is nothing special?

The first day you were listening to Kylie Minogue.  The next day you listened to Adele.

You know when a stereo sounds good when the instruments start sounding like instruments. When a guitar sounds like strings vibrating and wood resonating, and so on and so forth. When you walk by a closed door of a room where the system is playing and you're not sure right away whether it's someone inside playing live or a recording.

When I press PLAY and sound comes out of the speakers!