Purpose of Life, in my opinion, stems out from the basic belief that we have to serve Allah swt.
By serving Allah, means obeying his message, which was sent down to our Holy Prophet s.a.w., who in advance handed it over to our blessed Imams.
How we served is clearly outlined in their Sunnah, their teachings, and their actions in general. We do everything they have told us to do or did themselves. It is a broad range of doing obligatory actions such as first of all having firm beliefs in the 5 roots of religion (Usul e Deen)... and applying it through our Branches of Religion (Furoo-e-Deen) We MUST follow each and every one of them from Salat all the way to Tabarrah.
To do so, we need the guiding force of what the Quran tells us to do, as well as the sayings and teachings of the Ahlul Bayt.