God made us for a purpose and we exist for a purpose. To every man, there is a purpose. So I feel the reason why we exist is because we need to do good. The whole purpose of our existence is to do good and live a good life.
People are out there looking for help, troubled and frustrated. Some times we need to reason, why do God give you the ability and grace to make wealth and live good? It is to the reason that you help this ones who are needing and lacking. So, the reason why we are here is to help others, even in their lowers state of life.
I think by so doing, we are directly or indirectly fulfilling the plans of God towards our lives. Because God wants us to do so. So the reason why we exist, is to do Good.