Why does my background thread raise deadth of my UI？

Pop-UP dialog is my other common Form.

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
        }
        private void button1_Click(object sender, EventArgs e)
        {
            System.Threading.Timer tm = new System.Threading.Timer
                (
                (state) =>
                {
                    PopUpDialog ppd = new PopUpDialog();
                    ppd.Show();
                }, null, 0, 3000
                );
        }
    }

U know that Timer is a background-class, BUT IT MAKE MY UI DEAD!!!!!!

Why?

If I say:

this.Text = DateTime.Now("HH:mm:ss");

instead of

PopUpDialog ppd = new PopUpDialog();
ppd.Show();

My Dialog won't be dead.

Why?

Plz explain to me in details with reflected codes, welcomed.

Thx anyway!