Steven Pinker (whom I greatly admire) indicates in his book How the Mind Works that many of our mental stances are inborn, including whether one is liberal or conservative.
Let's for a moment accept this as a working hypothesis. Now, it's clear that one isn't born with genes for liberalism, so it has to be something else, something that is not liberal or conservative but which leads in one direction or the other. I'd think it is a way of looking at the world. I've long since decided that the fundamental difference between liberals and conservatives is their attitude toward the present. The conservatives like things the way they are (or were), and the liberals think we need to change things.
If this is so, then the whole reason for the vicious fighting between the two thought patterns comes down to the fact that liberals are born to view the world as imperfect and conservatives are born to view the world as fine the way it is.
Now, as with all things genetic, mental, and what have you, there is a caveat necessary. What our genes give is, in many cases, is not an absolute, overwhelming mandate (You must be a conservative), but a tendency. So, we could say that some people are born to tend toward liberalism, others are born to tend toward conservatism.
Since I am somewhat of a biological determinist, this makes lots of sense to me. It also makes it easier to forgive my liberal friends, and gives me reason to doubt both ends of the spectrum.
And, darn it, the half full/half empty glass metaphor is so after all.