Why does the US want Assad to leave?

Because the West has decided to remove the only obstacles from an excuse to fully invade the Middle East. First the West set up strong dictators, who in turn burtally suppressed their population which in turn increased extremism. Now the West has decided to support the extremists, in order to overthrow the dictators. Why? Once the extremist seize power and begin to cause chaos in the middle east, Israel will have the excuse needed to wage full out warfare on the countries. It may seem ludacris, but if you study history and know anything about the middle east, you know that this is exactly the kind of scenario which is bound to happen. I say let Assad stay, and should have let Ghadaffi stay as well.