怎样可以做到再添加时已经添加了的不会消失，，求大神帮忙啊

public partial class Form5 : Form
{
static string str;
static int i;
public Form5()
{
InitializeComponent();
}
public void Mn(string s)
{
str = s;
}
private void Form5_Load(object sender, EventArgs e)
{
/* i = listView1.Items.Count;
MessageBox.Show(""+i );*/
ListViewItem li = new ListViewItem();
li.Text = DateTime.Now.ToString();
li.SubItems.Add( str );//把获得的参数添加进去 但每次从新获得时已经添加的就会消失 ，这是为什么啊
this.listView1.Items.Add(li );
}

    private void button1_Click(object sender, EventArgs e)
    {
        Form2 fr = new Form2();
        fr.Show();
        this.Hide();
    }
}