Ok I give up: Why does wool have to be dry cleaned? Wool was far more popular back in the day, long before modern chemistry and dry cleaning equipment. How did say the US civil war armies wash their wool uniforms?
Why does water shrink/destroy wool? Shouldn’t wool love water? Dont it rain on sheeps?
I don’t get it . . .
I think it really depends on if the wool was knit or woven. I am a knitter and wool shrinks like no other fiber.
Woven wool is twisted tighter than knit wool so it is less-inclined to shrink and can stand up to a washing.
The big question is lining. Good wool suits and whatnot have lining which is why you usually want dry-cleaning; you are trying to clean two different types of fibers and washing them would affect them differently.
But I wash my un-lined wool garmets all the time. I am just careful with them; usually hand-washing and carefully pinning or hanging them dry.
But it doesn’t… my angora sweater got washed many, many times. Then Dad did the washing and put it in, with the lights as Og intended. But he used a high temperature - and my angora sweater became Barbie-doll-sized 
We’ve washed woolen scarves, hats, sweaters, rugs, socks, tights… in the machine with no ill results, but always at low temps.
As Fried Dough Ho said, it depends a lot on the specific cloth and on whether you have something that’s lined or not. Another problem affecting woollen suits is ironing: ruining a woollen suit (or semi-ruining it in a way that gets fixed by wetting it and drying it again, but you can’t just wet the affected spot) is relatively easy, the people at the dry cleaner’s know how to iron it properly.
I was once talking to a guy at the pub (Romano’s in Wagga Wagga) and he asked me to feel his hands.
Like a baby’s bottom they were. Soft and silky smooth.
He explained that it was because his job at the abattoirs was to throw the sheep carcass up onto the hook. The lanolin from the fleece kept his hands soft.
My mother used to hand knit stuff for all the family. She made her own wool wash from soap and eucalyptus oil and hand washed all the woolens. They lasted for years. Now you can buy wool wash at the supermarket and do your woolens on a gentle cycle in the washing machine.
I once put a king-sized wool blanket through the washer and dryer. It is now too small for even a crib mattress. (It’ll just cover, but you can’t tuck it in.
It makes a heckuva throw blanket though, and is about a half inch of thick felted wool.
The reason sheep don’t shrink in the rain is that the varieties kept in wetter countries (Wales, New Zealand and so on) have been genetically modified to be 50% nylon.
SmartWool™ socks are machine washable. Dunno if it is teh type of wool (marino) or the weaving or what.
Brian
Ever wonder how you get felt? You too can make felt out of your favorite sweater. 
ETA - I can’t quite come up with how to phrase this so as to not invite disreputable jokes. Oh well.
Cool or tepid water and a special wool cleaner such as Eucalan, then line dry, and you’re fine. Pants may have to be ironed, of course.
Some wool garments are made to be machine washable. Smartwool, for example. Even so, they last longer if treated as above.
Sheep don’t shrink in the rain because rain is a) close to the same temperature as the air, and b) not powerful enough to agitate the wool fibers. The best way to shrink a shrinkable fiber is to shock it with fast temperature changes while agitating it vigorously – which is a good description of what a washing machine does, especially if you wash with warm water and rinse with cold, as is default on many machines. (This is true for animal fibers like wool or angora and also for plant fibers like cotton, rayon, linen, etc.)
Many woolen items in olden tymes were made of pre-felted, or fulled, wool. Felted or fulled wool is mostly pre-shrunk and can take more laundering abuse than other woolen garments, such as your typical fancy modern tailored woolen pants.
I think part of the answer is that “back in the day,” people washed their clothes far less frequently.
To get a little more detailed, wool shrinks at all because wool fibers have scales like a snake or a pinecone. In their sheep-borne state, the scales are slicked down with the sheep’s natural skin oils (lanolin), just like your unwashed hair is eventually slicked down with your own skin oils (sebum). [Note: Technically, both lanolin and sebum are waxes rather than oils or fats.] When you do anything that removes lanolin, the scales are more free to stick up. When they stick up, they help the fibers stick to each other and “lock up” or tangle, like burrs.
Washing is a great way to remove lanolin: hot water liquefies it [since it is really a wax], soap breaks it up, and agitation gets the hot water and soap in and out of among the fibers efficiently. Once the lanolin is removed, agitation is also a great way to tangle wool fibers.
The individual fibers do not, as far as I know, change size – though with their scales sticking out, they are effectively thicker – but fabric made of those fibers does, since the tangles tie up more and more fiber into each square unit of fabric, with less and less give.
I had already heard that since wool stretched really well when wet, that washing ruins the “press” of the wool and the fabric is subtly misshapened
I hope the OP doesn’t mind if I piggyback a related question onto this thread.
Let us assume a person has a 100% wool felt fedora. This fedora has a cotton sweatband and satin lining.
How does one go about washing this hat properly and effectively?
You take it to a professional, who will steam it, brsuh it then reblock it. You can try the brushing yourself, it often works for me.
I don’t know, but I just inherited a new merino wool sweater when I tossed my BF’s into the dryer. I didn’t know it was wool; wasn’t paying attention. (Hey, he’d just moved in and I was washing everything that wasn’t bolted down that day.) I was all 
wow, honey, how did that happen? He said, I dunno, you want it?
Woot! New sweater for Dogzilla! 
(Glad I’m tiny!)
My question to you fabric experts. Now that I’ve shrunk my honey’s sweater down to Dogzilla size… what if I washed it again and then tried to re-block it while it was wet? Could I stretch it back out to BF size? (I doubt it’s likely I could stretch it evenly, so I’d probably ruin it, but I’m just asking.) Can we reverse this damage?
Well, only a little. Is ir one size too small or several sizes too small? If the latter, sorry.
For him or for me?
Fits me perfectly. 
I suspect, however, that we are several sizes apart. It’s a men’s medium and I am a women’s xsmall.
So great. I have a new sweater. I think I’ll wear it tomorrow. It’s gonna be chilly. I’ll get him a new one for Christmas or something.
What about wetting the sweater thoroughly and then putting it on him for the sweater to dry and take his shape? Perhaps he could put a few layers underneath to make himself thicker in the same way you put thick socks on to stretch a shoe to be comfortable.
If you do try that, whatever happens, post pictures.
That will likely ruin the sweater forever.
Even Dogzilla would be unable to wear it then, it’d be stretched out of shape.