Why does System.Threading.Timer stop on its own?

I'm doing a small test project before I use System.Threading.Timer in a Windows Service project. It's working wonderfully, however the timer stops on its own after a minute or two.

The full source for the test project is:

using System;
using System.Windows.Forms;
using System.Threading;

namespace studyTimers {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e) {
            TimerCallback timerDelegate = new TimerCallback(tick);
            System.Threading.Timer testTimer = new System.Threading.Timer(timerDelegate, null, 1000, 1000);
        }

        void tick(Object obj) {
            if (label1.InvokeRequired) {
                label1.Invoke(new MethodInvoker(() => tick(obj)));
            } else {
                label1.Text = DateTime.Now.ToString();
            }
        }
    }
}

The goal is obviously to update a label with the current time. I am noticing that updating stops after a bit. Why would this be?