Humans are frighteningly rational once they've selected their goals. They know exactly how to get what they want, be it drama or attention or destruction. The selection of those goals is also rational, but it depends on one's understanding of his role in the universe. As he sees himself as a part of a whole, his understanding changes and his goals change too.
Yes, we instinctively choose the shortest distance between two points. It doesn't matter whether we are out walking or implementing eugenics.
So the secret to life is to choose our two points carefully.
I think that for the most part humans are rationalising rather than rational, they find reasons after the fact, construct stories about themselves and others which are kind of personal narrations based on their inner dialogue.
There's a Robert Heinlein quote that I remember about humans being rationalising rather than rational.
This was my first thought when I read the title of this thread.
"Man is not a rational animal, he is a rationalizing animal." - Heinlein
It seems like we spend a great deal of time trying to discover the important things. In reality, we are frequently just gaining more understanding of why we do the things we do...because of human nature. In the absence of actually gaining more understanding, we make something up that is serviceably in line with our nature, like religion.
"You will always be fond of me. I represent to you all the sins you never had the courage to commit."
Reason is, and ought only to be the slave of the passions, and can never pretend to any other office
than to serve and obey them. - David Hume
I think that deep down, all humans are rational. Our incredible complexities help obscure the axioms on which those rationalities are based, and a lot of people make the mistake of applying their personal rationalities to other people.