Google creates its own links, but only for certain sites. As for how he determines which sites receive it and which don’t, I’m not sure, but I suspect that this is due to the site’s site and the amount of content that you have.
For a while I had links for my site (PR4 with 40,000 pages indexed by Google), but after a while they left. In my case, he created sitelinks for the main tabs on the site, probably because they are in the title navigation and therefore on each page at the top of the page.
The only control you have, you can use Google’s webmaster tools to remove sitelinks that you don’t like, but you can’t modify existing ones or suggest new ones.
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