In the beginning, there was keyword density, metadata, and other SEO tactics that were fairly easy to figure out and enabled you to dominate top search results. Sure, Google would continually tweak their algorithm and keep you on your feet. But achieving decent search results for your website was still heavily influenced by on-page tactics.
Then along came social media and, more recently, the Google Panda (aka “Farmer”) update. Almost overnight, SEO got a lot harder as Google got a lot smarter.