Web Forms :: Class-level Variable Loses It's Value?

Google usually solves my problems for me, but this one has me stumped. I have a page with the following code, in which lstPayments is a listview control:

public partial class frmPerson : System.Web.UI.Page
{
protected bool paymentWarning = false;
protected void Page_PreRender(object sender, EventArgs e)
{
if (paymentWarning == true)
{
string alertText = "alert('Hello World');";
if (!ClientScript.IsStartupScriptRegistered("Alerter"))
ClientScript.RegisterStartupScript(this.GetType(), "Alerter", alertText, true);
}
}
protected void lstPayments_ItemInserted(object sender, ListViewInsertedEventArgs e)
{
paymentWarning = true;
}
}

As you can see, I'm trying to get an alert box displayed after a new payment is entered. As I expect, lstPayments_ItemInserted is executed first, and paymentWarning is duly set to true. However, when Page_PreRender subsequently executes, paymentWarning has been re-initialised: it's false again!

The only way I can get the class-level variable paymentWarning to retain it's value between the two event handlers is to declare it static, which I don't want to do for obvious reasons. There must be something I am failing to understand about how class-level variables work in aspx pages.