Why is it mandatory to follow the sunnah?

I'm just curious since the Quran is complete, does it really need the sunnah? Could we call someone an unbeliever if they believed in the Qu'ran but not sunnah and if it is mandatory for us to follow the sunnah why didn't God promise to guard the sunnah against fabrication. I mean seriously theres like 300,000 sunnahs, telling you how to live your life to every little detail. Why would God leave out information in the Qu'ran, and put it in a source that is capable of being distorted. Also one time I remember seeing a sunnah inwhich the prophet knew who would go to heaven but God said know one knows who will go to heaven but him. I'm not saying the sunnah should not be followed, I'm not to say what is right or wrong but there are so many of them.... Will God be angry with you if you don't follow all of them?