How does a mortgage drive your net worth down? Sure you owe a lot of money to the bank for a while, but you have the value of the property to cover for it. Or

How does a mortgage drive your net worth down? Sure you owe a lot of money to the bank for a while, but you have the value of the property to cover for it. Or is it only for cases where the property value dives just after one bought it with a new - now over-estimated - mortgage? Usually banks perform their own evaluation of properties before issuing the mortgage.