Check out this screenshot; I've marked all mood changes for United America in the past few days of play.
The lowest one is a change from Pleased to Happy, after doing a mission. No problems here, as long as I understand it right that this is a step up.
The next one is a change from Happy back to Pleased at 00:01 the next day. Again, not a problem, since I assume this is natural decay and that mission earlier can't have gotten it all that far above the boundary between the two states.
The next one is an announcement two days later that happiness has changed back to Pleased. Now I'm a bit confused, since I thought it already was at that level - why does the game feel the need to tell me what I already know?
Then, 14 minutes (!) later, happiness drops from Pleased to Content. Okay, what the heck? Why did it drop more than a whole happiness level in just over two days of in-game time? It's not like I skipped a terror mission or anything - there weren't any missions that I was aware of since the first mood change in this log. I only have a tiny radar tower in North America, so I guess I could have missed a few UFOs flying around, but that's all.
After doing the next mission, it went back up, so at least it seems it didn't continue to drop at that rate. But I'd still like to know, what gives? Is there a random element to mood decay and I just got unlucky? If someone understands what happened here, please explain it to me.