How do you know when a stereo sounds good?

Any system i ever listened to was sounding good to at least ONE person or listener, the owner with or without my favorable opinion...

This is common place observation...

But reading the answers it seems nobody  had realized that almost all answers ONLY repeated an instance of this common place fact... Then these answers could never  said anything valuable about an objective criterion which anyway CANNOT and could not be "gear with good measurements"...Or the pricier gear....All gear at any price or  quality levels, NEED acoustic control and treatment at the end anyway....

The OBJECTIVE way to produce an answer is then by listening experiments in your own room with ACOUSTIC principles... Play with them.... COST=zero

Upgrading, even if the "upgrade " affected really positively some aspects of the sound experience, upgrading COULD NEVER rival or compensate for the impact of a negative or positive acoustic room settings...

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.