I just don't get this. Let's talk about Sanctuary just as an example.

22 settlers

25 beds, all indoors, spaced one bed per floor tile

36 food

182 water

162 defense, place hasn't been attacked since very early in the game

Level 3 shops, one of each, and the bar has a spacious, nicely decorated patio area to hang out in

I understand that this is not enough to get to 100% happiness, but why on Earth should the happiness drop from 87 to 47 in two game days? No attacks, no kidnappings, no raider or ghoul problems... nothing has changed.

From a story standpoint it makes no sense. With their three meals a day, warm beds and ring of turrets to live behind these jerks should be tap dancing in the streets. From a game mechanics standpoint it makes no sense either because, as I said, there's been no material change in their condition.

Can anyone who's hardcoe into settlement management give me some insight here?

You may be experiencing a bug that afflicts settlements. Are the values of the beds, food, water or power changing to numbers that are lower than they should be? Check the settlement in your "Data" tab of the pipboy. Look over everything to make sure the numbers are correct. If they are not, you probably have the same bug that I do.

The values should "reset" when you are actually in the settlement. The trick is to get them to stay at the correct value when you leave. Fast traveling from a bugged settlement almost guarantees that the numbers are incorrect, so once they are at the right value, try running away from the settlement until you are registered to be at a different location before you fast travel. This has proved to (temporarily) address the bug for me, although it appears to be permanent, and will eventually re-occur to any settlement afflicted by it.

If you are experiencing a different bug/issue, then I wish you the best of luck.

Oh, the numbers on the Pip Boy workbench report are all screwed up. That's a whole different rant

I've left settlements like Sanctuary, done some questing and come back to find happiness in the toilet when I get there even though all the other stats are the same. Also, I'm getting some dialog from random settlers that "people are complaining about the bed situation", when there's just no reason for that to be. Seriously, these people are living in mansions by Fallout standards.

I tell you, it's almost enough to make me just tear the place down and let them fend for themselves. There's a lovely pack of raiders in that middle school that would be happy to put them up.

When you spoil them. They can never be happy.

It has been my experience that if you give a town a tier 3 shop of every type, double food, craptons of water, and double defense (2x food+water)... happiness still just settles in an at 84 with occasionaly jumps by a few points for completing a quest that quickly drop back down. The pip-boy workshop view bug isn't cosmetic. For some reason those resources just stop processing and happiness will quickly drop because of it. As mentioned already, visiting the settlement and leaving the workshop zone on foot will fix the issue temporarily. The best way to avoid it is to never fast travel out of a workshop zone. Always leave on foot then fast travel. It still happens, but much less often. I think fast travelling to settlements can also cause the issue, but that is unavoidable without long journeys by foot.

If you want to get higher happiness than the 80s, you have to go the million clinics route. Everyone needs to operate a clinic except for the few people required to maintain food production and only make the minimum food required (1 per settler). Or better yet, add a mod that boosts the happiness production of clinics so you don't have to run Doctor Town for high happiness. Happiness is a bugged and nearly worthless mechanic.

I believe the pip boy workbench report is telling you the heart of the bug that is causing the problem. Your happiness is crashing because the game believes your assets to be the incorrect values listed in the pipboy. Those low numbers are the cause of the dialogue about "not enough beds" as well. It is all related to an issue with the game failing to register the correct values for those assets.

Here is a link to a thread with a video explaining how to fix the happiness issue: http://www.gamesas.com/topic/1571944-fixing-settlement-happiness-issues-a-workaround/

I think that video will help, as you are describing exactly what has been happening to me in my game. If you follow the workaround with all of the settlements experiencing the problem, over time, the happiness will stabilize to a correct value. It would be great if you could try it, and let me know in the thread whether or not you find that the workaround is working for you.

If it turns out to the bug is fast-travel related, that may help the developers QA team narrow down the precise causes and speed up a fix for it.

Shoot all of the settlers, 100% and 0% happyness at the same time.