Sometimes the reason can be very simple. Can you eat fish n chips 7 days a week? You need variety isn't it? :P
PS
But the same cannot be said of your wife. You do have to be with her 7 days a week. For variety I can think of can be C++ without references, C++ with references, C++ without pointers, C++ with pointers, C++ without inheritance, C++ with inheritance, C++ without composition, C++ with composition, C++ without STL, C++ with STL etc etc :P