I think maybe you are misinterpreting master pages.
For your described scenario, your master page, generally Site.master
should contain your menu markup.
Now, any pages you create (Default.aspx, why_choose.aspx) and use your Site.master
page will also include your menu, although the markup is not visible within these sub pages.
If you want to load some other content into these sub pages, you simply put that new content into the asp:contentplaceholder
on the sub pages.
Read up some more on master pages here