Value and Ref Types

Question

Hello All,

Why is that Structures in .NET are value based and Classes are reference Based,
I know that difference between value tye and Ref type and structures are best used for smaller, light weight ojects that do not persist for long, and classes are best used for larger objects that contain more instance data and expected to exist in memory for extended periods...
QUESTION:why is that classes are stored in both stack and heap where as structures with only stack????Plz can any one clear my doubt???