As I like to explain to folks at a settlement.  The mortgage is the encumbrance on the house and the note is the encumbrance on the persons borrowing the money.  Should you not be able to pay the mortgage and it is sold in a short sale or in a sheriff's sale, you are personally on the hook because of the note.  Usually a short sale approval letter will say if there are further amounts under the note that will continue to be owed by the borrowers following short sales.  Lenders want their money back, plain and simple.

The lender agrees to discharge to mortgage, but they may continue to collect their debt through other means through the note and any personal judgments they win in a legal proceeding.  The new owner gets a new house free of the old mortgage and the old owner has a continuing financial headache on their credit report, and on their next year's income taxes for the amount of debt forgiven in the short sale. 

Best advise: Pay the mortgage and you have no troubles!