From Google's 'Webmaster Tools' site (which you can log into to optimize your Google listing) [https://www.google.com/webmasters/tools/ ]:

Sitelinks are links to a site's interior pages. Not all sites have sitelinks. Google generates these links automatically, but you can remove sitelinks that you don't want.

Site links are completely automated and are displayed only if we think that they will be useful to the user. If your site's structure does not allow our algorithms to find good site links or we do not think that the site links are relevant to the user's query, we will not show them. However, we are always working to improve how we find and display site links.