To be blunt, Google doesn’t want to trust your website.
It takes a lot of effort to convince Google that your site is a reliable resource for anything and in the meantime they continue to hunt for site issues in increasingly complex ways. One of the strongest ways to show Google that your site is a trustworthy and valuable resource is through the cultivation of quality backlinks. Links that point back to your site provide a sort of third party validation. Google rightly assumes that if trustworthy and valuable sites are pointing to your site, you must be trustworthy and valuable too.