Why does the Form_Paint event run continuously and only draw

Question

I have the following code, I put a label in Form_Paint to track the number jump, I see the label jumping continuously but e.Graphics.DrawString(...) only looks once, does anyone know why ?

public partial class Form1: Form {
 public Form1() {
  InitializeComponent();
  //progressBar1.Visible = false;
  progressBar1.Minimum = 0;
  progressBar1.Maximum = 100;
  this.Paint += new System.Windows.Forms.PaintEventHandler(this.Form1_Paint);
 }
 int iNum = 0;
 private void btnDrawText_Click(object sender, EventArgs e) //PaintEventArgs e
 {
  progressBar1.Value = 0;
  progressBar1.Visible = true;
  this.timer1.Interval = 100;
  this.timer1.Enabled = true;

 }
 private void ShadowedTextPaint(PaintEventArgs e, int num) {
  using(Font font1 = new Font("Times New Roman", 250, FontStyle.Bold, GraphicsUnit.Pixel)) {
   PointF pointF1 = new PointF(310, 270);
   e.Graphics.DrawString(num.ToString(), font1, Brushes.LightGreen, pointF1);
   lblNum.Text = num.ToString();
  }
 }
 private void Form1_Paint(object sender, PaintEventArgs e) {
  ShadowedTextPaint(e, iNum);
 }
 private void timer1_Tick(object sender, EventArgs e) {
  if (progressBar1.Value < 100) {
   Random rd = new Random();
   iNum = rd.Next(0, 999);
   progressBar1.Value++;
  } else {
   this.timer1.Enabled = false;
   progressBar1.Visible = false;
  }
 }
}