Wool is great at keeping you warm, but it's not so great at surviving a ride in your washing machine. This video explains why shrinkage isn't the only problem you have to worry about.
You're probably perfectly aware that wool comes from sheep, but as Hang Green explains in this video from the SciShow YouTube channel, you may not realise what it means to have a sweater essentially made of protein. The "shrinkage" that happens when you wash wool is something most people know about, but it isn't really shrinking at all. It's the fibres clinging together so tightly the wool appears to have shrunken down.
The other big issue with washing your wool is your laundry detergent. Modern laundry detergents have enzymes that are specifically designed to break down other biological molecules. These enzymes do wonders for removing food stains and whatnot, but they also break down the molecules in your wool. So, if you have to wash your wool items, do it in cold water with no agitation and detergent designed for use with wool. If it's a blanket or scarf, a little fresh snow can do the trick as well. Or you could always just take it all to a dry cleaner.