why do moods seem to swing so dramatically?

Around 7:00 PM tonight, after a very difficult year and a very very difficult month, I had some profound self-acceptance. Perhaps more than I have ever had in my life. I was fully content with everything that I had. I had no desire to create conflict in any way whatsoever.

Having done a lot of mental health work today, I laid on the couch to watch some TV, hoping to wind down.

By 9:00 PM, I was having this fantasy of some very immature temper tantrum revenge against a former employer. I dialogued with that part and eventually he agreed to not engage in that childish revenge.

But my question is: how did my inner peace change so dramatically in such a relatively short period of time?

Why did that feeling of deep self-acceptance fade so much?