Since you are already alive, why not make full use of your lifetime to do what you want? Having this mindset allows you to make use of your precious life time to do what you REALLY want to do.
It's a very interesting question and could be looked at from two different ways.
First for those who believe in the life hereafter. For them the answer is simple. They are to be rewarded for there acts in this world, good for good, bad for bad but the mercy of God would prevail for those who did not make companions with God and believed in one and only one God. So there is lot of incentive to live a life believing in God and spending this life as He desires with reverence and being kind to the fellow human beings in a just manner.
For those who do not believe then they have the incentive of achievement if they make any but this is limited to their life because they are going to die any way. After their death will never know whether there fellow beings appreciated their achivements ore good deeds after one has died or they will be remembered or not. Hence in a way from the non-believers point of view, it might not be worth living for a short span of few years in this world.
Conclusion is obvious - live with a belief in God and hereafter to harvest the fruit of your good deeds. It gives you hope and desire to live a good life.