Have you ever hated the sound of your system one day, and loved it the next?

Yes.  Can sound amazing one day and (infrequently) irritating the next; way more good days than bad, fortunately.  
Two variables I attribute this to are:
- weather conditions, esp. humidity (I haven't kept a log but suspect rain and or/higher humidity HELP with sound from my gear);
- how well-rested I am.  
There may be other causes though I do not associate performance differences with a specific time of day or day of the week - possibly because we live in a fairly low population area.

Timely tip. Keep a log. 

It's you.

(By far the most reasonable explanation).

I don't have quite the swings of "love it/hate it" but my system DOES strike me sometimes as starting to be unsatisfying in one way or another, and if I just stop listening for a little while (few days, whatever), bam it blows me away again.

(Or just tweaking the speaker position).

Most claims of "break in" are people getting used to the sound of their speakers, IMO.

Absolutely not!  My system just doesn't sound terrible on some days.  On some days I get less involved when listening to music, but on some days food doesn't satisfy as well as on others.  I agree with what Prof said above, it's you.  Stress, sleep patterns, diet, sex life, noise exposure, bowel movements, drugs (both legal and others) and biological rhythms can all have a serious effect upon your ability to enjoy the sound of music.

Whoa! Hey! I never thought I’d live to see bowel movements and sex life in the same sentence. What the ding dong?!