OnCheckedChanged event doesn't work

<asp:CheckBox ID="checkBox1" runat="server" OnCheckedChanged="checkBox1_CheckedChanged" Text="I agree" /><br />
<asp:Label ID="label1" runat="server" Text="Check the check box if you agree." />

protected void checkBox1_CheckedChanged(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        if (checkBox1.Checked)
        {
            label1.Text = "You agree. Please click on the button to go to the next page.";
        }
        else
        {
            label1.Text = "Check the check box if you agree.";
        }
    }
}