Direct democracy is not always desirable. When people are directly affecting their government, it can get chaotic, and nothing will change. If there are a lot of people attempting to do different things for their government, it is hard to get any thing done because the government is being pulled in so many different directions.
No, direct democracy is not always desirable, because a direct democracy is more or less mob rule. If laws change based on the whim of the people, there will be wide swings in policies that a functioning government cannot withstand. In addition, it is very inefficient to have everyone in the democracy vote on everything. In any system of considerable size, it would not be feasible to have that many votes on every issue.