CheckBox.CheckedChanged not fired on winform

Using windowsform: CheckBox.CheckedChanged not fired on winform on newest questions tagged winforms – Stack Overflow

 public Form1()
    {
        InitializeComponent();
    }

    private void checkBox1_CheckedChanged(object sender, EventArgs e)
    {
        textBox1.Enabled = checkBox1.Checked;
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        checkBox1.Checked = false;
    }

I set the check state in load event handler, and why in this case, the CheckedChanged not fired?
If i click the check box, then the CheckedChanged is fired.

source: http://stackoverflow.com/questions/11878527/checkbox-checkedchanged-not-fired-on-winform
Using windowsform: using-windowsform