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

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.

Short answer
Yes

No, not ever. Not enthusiastic at times but never hate.