Good answer Don. As a further extension I also feel compelled (sort of a pun there) to add that there is also the slightly less agreeable (but still true) implication that we are "able to leave when the play has ceased to please us" -- meaning that under truly horrible circumstances there is always the possibility of taking our "final exit."
And yes as I was reading the question I was also thinking that the question was probably aimed at the social implications, but i think it is pretty clear that there is no divine or other conscious intention behind what is. As ccaruth said, these differences just "are." Probably in this context there are other Lucretian passages worth consideration, in that he discusses the personalities / characteristics of different types of animals as arising from the different types of atoms and ways they are combined. That would no doubt apply to humans as well, even though we have more discretion over what to do with our atoms than lower animals do with theirs.