Bitcoin becomes currency when it becomes more widely adopted. The reason most think of it more like stock than an actual currency is because the adoption of its use has been limited due to it being so early in its life. As long as people believe in it, buy it and hold it, work for it or trade it, it will continue to be worth something. Furthermore, it is a revolutionary technology, that just so happens to specialize in transaction. That gives it its inherent value as currency.