Former Ambassador Dan Simpson has written an excellent article on why Afghanistan is lost and Obama should get out:

It is simple. It is impossible to fix Afghanistan without fixing Pakistan. It is impossible to fix Pakistan. Thus, Afghanistan is and will remain an impossible sinkhole

I’ll go one further.  The war in Afghanistan is destabilizing Pakistan.  Continuing or escalating the war in Afghanistan may not just mean Pakistan isn’t “fixed,” it may mean that Pakistan falls into anarchy or into the hands of Pakistani Taliban or other folks fundamentally hostile to the West.

In any case, go read Simpson’s full article, he makes his case very well.