Garry: Every time another mass shooting takes place, we immediately wonder: Who did it? Why? It’s human nature. We want answers to help us make sense of the senseless. Numbed by the unfolding details of the slaughter, we sit in front of our TVs listening to reporters speculate about the killer’s motives, or we turn to social media, looking for clues, sharing updates. Media alerts flash on our smartphones, such as these this morning from The New York Times: “The …