Thoreau believes that many people have their priorities in the wrong places; he feels that our lives are cluttered with too many things, "frittered away be details" and that we should "simplify, simplify, simplify!" Along these lines, because many people's priorities are out of whack, they are unhappy, but in such a way that they aren't even really sure why or how. So, they are quietly surviving each day, desparately searching for happiness, but not finding it. The unhappiness is subtle and non-obvious (hence, quiet), and their searching for happiness comes in a desparate search for something to fulfill their lives. Think of how many people you know that move from one thing to the next, constantly staying busy, constantly filling their lives with new things; perhaps, Thoreau is saying, that is because they are quietly desparate for a fulfillment that is missing in their lives. Thoreau's solutions are pretty extreme; he feels that stripping our lives of all unecessary activity, comfort and privelege is the way to be fulfilled. He feels it is the only way to "live deep and suck the marrow out of life," and that it will help us to not be so quietly desparate about our situations.
Those are just a few thoughts; I hope that they help. I provided links below that lead to some great discussions of "Walden" and the themes contained therein.