Consider a asp:dropdownlist which fires SelectedIndexChangedEvent based on listitem index..... 
But why the event doesn't fire for the same listitem index when selected again....
Consider a asp:dropdownlist which fires SelectedIndexChangedEvent based on listitem index..... 
But why the event doesn't fire for the same listitem index when selected again....
Because it didn't change.
You can solve this by resetting the value after each change.