The value of DataGridViewRow.Tag is lost after sorting

I'm facing one problem with DataGridViewRow.Tag. I set some values to DataGridViewRow.Tag. Then, I get those values whenever it is necessary. but when I do sorting the GridView by clicking the header of Girdview, all values which was set are gone...

The following is my sample code..

Filling the data to DataView

private void Form1_Load(object sender, EventArgs e){
 this.employeesTableAdapter.Fill (this.employeeMgmtDataSet.Employees);       
}

Setting the value to DataGridViewRow.Tag
private void button1_Click(object sender, EventArgs e){
  foreach ( DataGridViewRow dr in dataGridView1.Rows ){
    dr.Tag   = "Michael Sync";
  }
}

Getting the value from DataGridViewRow.Tag

private void button2_Click(object sender, EventArgs e){
  foreach ( DataGridViewRow dr in dataGridView1.Rows ){
   Console.WriteLine (dr.Tag.ToString ());
  }
}

it works fine before sorting.. but Tag become "null" after sorting.. Why is it happening like that? Any idea would be appreciated. Thanks in advance..