Default values of instance variables and local variables

Standard local variables are stored on the stack and aren't actually created until they are initialized. If a local variable isn't used, it doesn't go on the stack. Member variables, however, are allocated in the heap, and thusly have a default placeholder (null reference or default primitive).

The non-technical reason behind may also be the following one:

If you declare a local variable, you do this in order to use it. And usage is connected with assigning a value. Therefore, accessing a declared, but not initialized, variable does not make that much sense - the programmer might simply have forgotten to initialize the variable.

Fields, however, might only be used until or after a specified point in the object's lifetime. Forcing the programmer to initialize them all would not be good.