Why did President Lincoln want to prevent Maryland from seceding?

The most important reason that President Lincoln did not
want Maryland to secede from the Union had to do with the location of Washington, D.C. 
Maryland lies to the north of Washington.  This meant that, if Maryland were to secede,
the capitol of the United States would have been cut off from any contact with the US
and would surely have fallen to the Confederates.  This would have been a humiliating
thing that would have helped the Confederacy
immeasurably.

Of course, Maryland was also important just
because of the fact that it had a relatively large population and would have added to
the manpower and industrial capacity of the Confederacy.  However, the most important
reason why Maryland, in particular, was important was its geographical
location.