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.
    <form id="form1" runat="server">
    <tr><td>Mass Add Tool</td></tr>
                <td>Asset Number</td>
                <td>Asset Name</td>
                <td>Purchase Order</td>

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

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


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

            GridView1.DataSource = dt.DefaultView;


        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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