In asp:checkbox, OnCheckedChanged event doesn't fire when check the checkbox

This question is a little bit tricky. There is a nested repeater in my work. I'm responsible to add asp:checkbox on every underneath repeater item, so that we can control each item by checkbox.

The markup code is:

<asp:CheckBox runat="server" ID="checkBox" AutoPostBack="true" OnCheckedChanged="CheckedChanged" />

The c# code is:

protected void CheckedChanged(object obj, EventArgs e)
{
    // some works    
}

I start to work on this problem and find the CheckedChanged funcion isn't fired via debugging. But, when I add a line in

if(IsPostBack){
   CheckedChanged(sender, e);  //add
}

It works and goes into CheckedChanged function when I debug. I have read many articles, none of them says I need to add that line in IsPostBack block. Is there anyone can show me the principle of that, thank you.