When an HTTP request to show a page is sent to the Stack Exchange web site, a check is explicitly made to ensure that returned page is not loaded into an
<iframe>. The response header that comes back for
SAMEORIGIN which means that only pages loaded from the same origin can be included in the
<iframe>. There are good reasons for restricting pages from being loaded within
<iframe>s and Stack Exchange have deliberately chosen this policy.
The bottom line is don't try and load pages from Stack Exchange within your own
<iframe>s, they won't work by design and intent.
To read more about the X-Frame-Options response, read the documentation and the technical spec in RFC7034.