Why can't I view --> page-source of an iframe? How can I make iframe contents viewable?

When I load a page that has an iframe, often (perhaps always) I can't see the content of that iframe using the browser's View --> PageSource menu choice. Two questions:

1. Why should the iframe's content be unviewable?

2. How can I change my html or css so that it is viewable?

EDIT: @diEcho and @JohnP answered the second question (thanks, guys!) but the first question remains:

How come the iframe's content is unviewable in the first place?

Thanks!