Why does ModalPopupExtender closes on Postback

I am using ModalPopupExtender to display a panel (with data entry form in it). The problem is that it closes on every postback. Although I can keep it open using ModalPopupExtender.show () method, but I don't like this approach. My questions are:

1. Why does the modal closes on every postback.

2. I have looked into a few hacks to keep it open but can't this behavior be fixed?

Thanks...