On which event or how should I bind data to the dropdownlist to avoid selected value overridden?

For some reasons, I can't use "IsPostBack" to bind data only on postback

I've tried binding data on page init, it works ok but when postback, the selected value will be overridden (Loss)

          protected void Page_Init(object sender, EventArgs e)
         {
            this.BindData();
         }

          protected void BindData()
         {
           //grab the source of dropdownlist
         }