Whatever the reasons for the adoption of the theory of rebirth without questioning it, the Buddha firmly maintained that was why birth occurs. That is, the basis of the present life is that of the previous ones. The desires one forms and acts out in those lives remain with one and manifests in the following ones. In that sense the desires are the upadana or the foundation on which the lives are built upon. The Jain’s who had developed a whole system of philosophy prior to this period had a curious notion about ‘karma sarira’ or desire body that is observed in company with our bodies. This body is said to be attached to us permanently and is the sum total of the desires and thoughts we harbor inside. It goes without saying that this body is as physical as everything else is.
So the cause of misery is the birth and birth is caused by previous births. These births as well as the present one are the result of desires that become attached to our consciousness because of their ‘desire’ to go on living!