Dropdown value disappears after submit

I have a textbox whose values goes into the dropdown on button click. The problem is that when I fill all the data and submit the form. And when I come second time to see that value it disappears from the dropdown. What should I do to make the value gets in the dropdown fix. Please see the code for your reference:
<tr> <td class="td">Location/City</td> <td> <asp:DropDownList CssClass="txtfld-popup" ID="ddlLocation" runat="server" AutoPostBack="true" OnSelectedIndexChanged="ddlLocation_SelectedIndexChanged"></asp:DropDownList> <asp:RequiredFieldValidator CssClass="error_msg" ID="reqLocation" ControlToValidate="ddlLocation" runat="server" ErrorMessage="Please enter location" InitialValue="--Select--" SetFocusOnError="true">& ...