Posted 06 December 2012 - 07:56 AM
They're not obstacles precisely. The whole of samsara is an obstacle.
A soul in samsara is in a cycle of being reborn. Every time it dies it gets a new physical form which is determined by its karma from its previous existence. A soul with bad karma is sent down to the lower forms until its bad karma is worked off essentially, then it gets reborn as a human. A soul with good karma is reborn as a deva or an asura, until its good karma gets used up and it gets reborn as a human.
The lower forms are not capable of higher thought and so cannot contemplate the nature of suffering, and thus are incapable of reaching nirvana. The higher forms do not experience suffering, and so are incapable of contemplating it and reaching nirvana. But truly understanding the nature of suffering is very hard even for a human, so most people get reborn.