As I frequently observe, several commandments referring to social justice -- such as the command no to oppress the stranger -- refer back to the experience of being slaves in Egypt. "For you were strangers in the land of Egypt." If the experience of slavery and oppression causes one to have sympathy with others, why does God feel the need to emphasize this?
Because the experience of oppression does not "naturally" create empathy. More commonly, the experience of oppression creates the opposite; it creates a license to ignore oppression, or even to engage in oppression. "Why should I feel any sympathy for you when what I went through was so much worse?" Therefore God must command us to learn empathy from our experiences.
I also liked this post on 538.com on the Myths around Prop 8, especially his debunking of the idea that it was the "Obama surge" that defeated Prop 8.