Complex motivation

The class discussion about Hurricane was really interesting to me, but we still concentrated on ourselves--we are considering nature, but considering in a way that focused on how to change the behavior of ourselves in long term or short term.

I was about to leave this topic behind when I read Jan Narveson's article "on the survival of humankind", in which he pointed out that we have no OBLIGATION to think about the lives of our next generation. In addition to this, he analyzed the relationship between our happiness, number of human beings and prolonging the existence of our species on earth, and stated that even if we do many things nice to the nature, a huge, unexpected natural or universal disaster could bring the whole species to an extinct.

So what I was wondering was--do we have the obligation to prolong the existence of the species? If not, what motivated us to take action to think about it? Do we do everything based on "rational" decision? In economics, "rational person" means a person who maximizes his or her personal benefits.