AJAX :: DropDownList Selected Value Lost When Populated From Code Behind

When I use the code below an error accur when click add button to insert into sql because the selected value in DropDownList is back to --select-- Idon't know why it should be the the value I selectedbut when not use this code and bind the DropDownList using wizard it works but I need to use the code to make the first selection is --select--.

private void PopulateDropDownList() {
string constr = ConfigurationManager.ConnectionStrings[1].ConnectionString;
using (SqlConnection con = new SqlConnection(constr)) {
using (SqlCommand cmd = new SqlCommand("SELECT Country_ID,Country_Name FROM Country", con)) {
using (SqlDataAdapter da = new SqlDataAdapter(cmd)) {

[Code] ....