Beyond purely 'rational' choices:

"Now, let us think of the new mammalian brain as having evolved as a sort of portable computer, an information-processor capable of doing simulations with great speed and clarity. It is 'programmed' to calculate the path of least pain and greatest pleasure. But, as we have seen, the resulting calculations at times fail to enhance inclusive fitness. At such times, selection will favor the 'overriding' of the new mammalian brain. ... We experience these overrides, subjectively, as emotions. This does not mean that this is all there is to the emotions or that they serve no other functions, of course, but they do seem to be associated with what is tempting to think of as limbic-system overrides of the neocortex, of the old mammalian brain overriding the new."

Jerome H. Barkow, 1989