It seems as though the purpose behind Buddhism and their basic reasoning behind what they do and what they believe can be traced back to the four truths.
All existence involves suffering.
This can be found in everything. Duhkha is an obvious part of human life, and I would think that is even more relevant when you are talking about the part of the world that Buddha was speaking of. Trishna is another common thread found through religions, but once again, the life that the Buddhist gives himselft to a hard one. They obviously believe that they ability for a human being to get rid of their human wants and desires, is the ability that they must prey upon. Nirvana, or the release from suffering, comes through ending passionate cravings.