Modal Popup Issue

Question

I am creating a Modal Popup page(ModalPopup.aspx) on click of a Link Button from Javascript.

popupURL = popupURL + id(Query String Message);

var modalFeatures = "center: yes; dialogHeight: 675px; dialogwidth: 750px; dialogTop: 25px; dialogLeft: 150px; center: Yes; resizable: NO; Menubar: No; dialogscrollbars: NO;";

var returnValue = window.showModalDialog(popupURL, 'NewModalPopup', modalFeatures);

For the first time if I am clicking the Link Button it goes inside the page load event of ModalPopup.aspx.

If I am closing it and again clicking the Link Button then it is not going inside the Page_Load event of ModalPopup.aspx.

I don't understand why it is not going inside the Page_Load method.