Unfortunately, since -- as you point out -- a home is nearly always a family's largest expense and often also largest store of value, people distort what it means to them financially and expect their home to appreciate in value well above and beyond inflation.
When you take out a mortgage to buy a home, you shouldn't have any preconceived notions about whether or not you'll be "underwater" at some point in the future. And really, it does not matter one bit if you're underwater. Unless something has also happened to your income stream, you just keep paying it, as you should. The borrowed money doesn't somehow magically become less money just because conditions have changed and your home's market value has dropped. The downside is, of course, that you may not be able to sell the house and move away until you've paid off more of your mortgage. But that's a risk you take when you sign that paperwork up front.
The home-as-investment thing is fairly uncommon in the world and seems to mainly be a US thing. I wish it would die, as it should. It only serves to distort the market, and fuels ever-increasing home prices, which makes it continually harder for the average family to own their own home, further increasing institutional ownership.
It's unfortunate that this appreciation has progressed to the point where nearly all of a family's wealth is tied up in their home, as it is not a risk-free asset to hold.
People want _house_ values to go up, but _home_ prices to go down. And those are two different things.
If it's your home, who cares how other people value it. Lesser price is better.
If it's a house that you don't consider home, then you only want to sell it for what you can.
Personally I think home-owners > house-flippers and that laws should reflect that.
Anyone who bought at these prices is already underwater. The thing to look at is housing as a multiple of median income. If it looks insane, it is insane.