Basically, it is out of the Search engine optimizer's control to enable sitelinks in organic search result. Google has a process that produces these sitelinks which is executed through its algorithm based on various criterias. This typically occurs when someone searches for a brand or a company name in Google. It should be understood that not all sites get these sitelinks, it occurs mainly for websites that are huge, meaning sites with multiple subpages which should also be indexed by Google robots. Few of the criterias that enables these sitelinks are - the navigation format, the main navigation links and the popularity of these links. So it is obvious that one has no control which links of your website will appear in a sitelink.
If you had guessed 'relevance' also plays an important role, then you are right.
Hope this information was useful to people who wanted to know what sitelinks was.
Cheers!
