trying to bind grid to datatable but i keep overwriting row 1

Every time I press the submit button it replaces the grid with only one row from datatable.
Capture.JPG
<body>
    <form id="form1" runat="server">
    <div>
<table>
    <tr><td>Mass Add Tool</td></tr>
    <tr>
        <td>
            <table>
            <tr>
                <td>Asset Number</td>
                <td>Asset Name</td>
                <td>Purchase Order</td>
                <td>Vendor</td>
                <td>Manufacturer</td>
                <td>Model</td>

            </tr>
            <tr>
                <td><asp:TextBox ID="TextBox1" runat="server"></asp:TextBox></td>
                <td><asp:TextBox ID="TextBox2" runat="server"></asp:TextBox></td>
                <td><asp:TextBox ID="TextBox3" runat="server"></asp:TextBox></td>
                <td><asp:TextBox ID="TextBox4" runat="server"></asp:TextBox></td>
                <td><asp:TextBox ID="TextBox5" runat="server"></asp:TextBox></td>
                <td><asp:TextBox ID="TextBox6" runat="server"></asp:TextBox></td>
            </tr>
        </table>

        </td>

    </tr>
    <tr><td><asp:Button ID="Button1" runat="server" Text="Save Row" OnClick="Button1_Click" /></td></tr>



</table>
        <asp:GridView ID="GridView1" runat="server"></asp:GridView>


    </div>
    </form>
</body>


namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {

         DataTable dt = new DataTable();


        protected void Page_Load(object sender, EventArgs e)
        {
        }

        protected void Button1_Click(object sender, EventArgs e)
        {

            DataRow dr = null;


            //define the columns
            dt.Columns.Add(new DataColumn("AssetNumber", typeof(string)));
            dt.Columns.Add(new DataColumn("AssetName", typeof(string)));
            dt.Columns.Add(new DataColumn("PurchaseOrder", typeof(string)));
            dt.Columns.Add(new DataColumn("Vendor", typeof(string)));
            dt.Columns.Add(new DataColumn("Manufacturer", typeof(string)));
            dt.Columns.Add(new DataColumn("Model", typeof(string)));


            //create new row
            dr = dt.NewRow();


            //add values to each rows
            dr["AssetNumber"] = this.TextBox1.Text; ;
            dr["AssetName"] = this.TextBox2.Text;
            dr["PurchaseOrder"] = this.TextBox3.Text;
            dr["Vendor"] = this.TextBox4.Text; ;
            dr["Manufacturer"] = this.TextBox5.Text;
            dr["Model"] = this.TextBox6.Text;


            //add the row to DataTable
            dt.Rows.Add(dr);


            GridView1.DataSource = dt.DefaultView;
            GridView1.DataBind();

            ClearTextFields();
        }

        private void ClearTextFields()
        {
            this.TextBox1.Text = "";
            this.TextBox2.Text = "";
            this.TextBox3.Text = "";
            this.TextBox4.Text = "";
            this.TextBox5.Text = "";
            this.TextBox6.Text = "";

        }
Steve HougomDeveloper IIAsked:
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Steve HougomDeveloper IIAuthor Commented:
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Steve HougomDeveloper IIAuthor Commented:
fixed my own solution I wasn't saving my dt to session.
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