We have been born billions of time in all imaginable forms and in all imaginable places. We have been worms, ants, cats, monkeys, elephants, humans, gods and all the other beings for billions of times. In all these forms we have experienced suffering and pleasure. And the desire for more pleasure becomes the fuel for continued existence and hence the wheel continues to move. 
As long as we continue to exist there is no end to our suffering. From the time of our birth till our death there is always suffering. We blindly believe that the acquisition of people, wealth and fame will put an end to our suffering. But do we really find true happiness in them? We continuously and foolishly try to satisfy our desires until we become completely disillusioned by this. Then a sudden realization comes that there is no true happiness in the world. True happiness comes from transcending the world.
What is the cause of this suffering? It is ignorance. The world continues to be pleasurable and painful because of this ignorance.
Ignorance of what? Ignorance of our true nature. This world appearance veils our vision from the truth. With insight, it is easily seen that nothing in this world can give you pleasure and what appears to be pleasurable is actually a source of pain.
There is no pain or pleasure that has not been experienced. All experiences are transient. There is no permanency in this world, everything is changing whether one likes it or not.
One can realize that this existence is futile with insight, insight into the true nature of the world and one self. Once this is realized then one is automatically in a higher level of understanding and becomes eligible for the next process - realization of the truth. Until then this world will continue to be a source of both pleasure and pain.
