If next life is based on the deeds of current one then why not the current life be based on the previous one?

We are instructed in all our religions to refine ourselves and do only good deeds. Bacause the next life (janma) is determined based on the credits gained or lost in this life. So the logic says that there were previous lives and that resulted in this birth. Since God is the most intelligent and logical being one cannot say that He behaved randomly. So why deny previous birth?