Why is string nullable? [duplicate]

Question

Possible Duplicate:
The type ‘string’ must be a non-nullable type in order to use it as parameter T in the generic type or method ‘System.Nullable<T>’

As the title says, why are strings nullable by default in C#, but if I want, say, ints or doubles to be null, I have to explicitly say so?

Answer 1

Because string is a reference type, deriving from object and [most] other default types are value types, deriving implicitly from System.ValueType;