Why do Muslims strive to follow the Sunna?

Well sunna was the first form of Islam and Prophet Mohamed, who is the main religious figure in Islam was a Sunni. People liked to follow Mohamed as they overall saw him as a very good person and wanted to follow his ways. Shia was later on developed and some believe that it is not as good as Sunni as the Holy Quran book, it states that real tattoos are not allowed as you are harming yourself and it is not healthy to the body. Shia tends to break those types of rules while Sunni follows them. Shias during a time of the year torture themselves because an important mans death which is incorrect because one should never harm themselves.