When one talk about sunnah, he or she needs to talk about the Prophet, Muhammed's deeds, words and the things that he agrees about or the things that he leaves open. This means that sunnah is the second source of Islam. In this part, the Prophet explains everything concerned the Holy Quran. He also guides his followers towards the good deeds. By doing all what the Prophet, Muhammed, says, people will do what Allah wants or asks for.
Of course when doing what is sunnah, Allah will reward you. But if do not do what is sunnah, like performing two Rukaas after Zhur Salah, you won't be rewarded or punished. Therefore, to feel happy and content, you had better do what is called sunnah.