why is my dropdown list getting reset after the page refreshes?

I have a created a form using a user control.  One of the dropdownlists in the form is populated after the user makes a specific selection from another dropdownlist...i.e. a dependent dropdown list.  This is triggered using the SelectedIndexChanged event handler on the controllling dropdownlist. The dependent dropdownlist gets populated correctly, but when I choose one of the options and go to submit the form, the dependent dropdown list is reset and doesn't maintain the selection that was made. The controlling dropdown list ID is "level".  The dependent dropdown list ID is "EnrollmentSize".
The "EnrollmentSize" DDL only losses the option that was selected.  The DDL is sitll populated with all the correct values.  It only losses the option that was selected and goes back to the first option...i.e.  "Please Select..." The viewstate on the page is enabled and al ...