Populate Multi Line Text Box From Down List?

What i have on my asp.net web page is a drop down list a add button and a mutiline text box, what I am trying to do is when user selcts a item from the drop down list it should populate in the multiline text box, I am able to achieve this partially with the multiline text box only showing the latest selection it does not remember the previous selection. Listed below is the code behing page

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
jtOperationsddl.DataSource = sqlServer.GetOperationsForDataEntry();
jtOperationsddl.DataValueField = "OperationID";
jtOperationsddl.DataTextField = "OperationName";
jtOperationsddl.DataBind();
}
}
protected void btnAddOperaions(object sender, EventArgs e)
{
listofOperations.Add(System.Convert.ToString(jtOperationsddl.SelectedItem));
String s = null;
for (int i = 0; i < listofOperations.Count; i++)
{
s = s + System.Convert.ToString(listofOperations[i]);
}
jtOperationsmltbx.Text = s;
}

Every time I hit the add button the List becomes empty