What causes butter to make risotto shiny?
There is no special chemical reaction. Melted fats are glossy, look at any oil in a bottle. 

When you add the butter, it coats the rice, and this coat of fat is glossy. That's all there is to it. 

I don't know the exact explanation of why all oils are glossy, but it probably involves quantum physics. You could ask it on Physics or Chemistry SE, it is beyond our scope here.