Often methods are used to modify the attributes (or state variables) of an object. These methods do not return a value to the caller. Therefore, the only way to test that the change of state has been correctly performed is to send additional messages to the object.
Even if a method has been adequately tested, the same method may still require thorough testing when inherited, unchanged, by a subclass.
However with proper design the use of the object-oriented paradigm does reduce the need for testing.