Java script popup works with asp.net button but only when pressed twice?

 I have an asp.net web form with an Asp.net style button. When it is pressed, I want to display a Java Script
 popup dialog box with a message. The code works displaying the popup dialog box but only after I press the asp.net
 button twice?
 The first time the button is pressed, my subroutine is executed (See code below) and the dialog box does NOT display.
 The second time I press the button, the dialog box does indeed display, and I noticed that the code below is NOT executed twice.
 So why do I have to press the Asp.net button twice before it will display?

   Protected Sub Button1_Click1(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim thing As String
        lblCustomerCreated.Text = lblCustomerCreated.Text & txtBoxCustomerTitle.Text & " Has been created ...