“Sontarans are great… I think that’s partly because they come from a very specific world. That back story gives them a great context. Robert Holmes didn’t just create a race, back in the 1970s: he created a world that they came from. Even if you never saw that planet, you understood why they did what they did.”—David Tennant, Doctor Who Magazine #395
I hate to contradict our Time Lord and Savior, but he’s ever so slightly off in his close. It’s not that we ever understood why the Sontarans did what they did, but rather that Holmes’ vision of the race was so clear that we accepted what they did without question—and that was make war (not love). The “why” could make for a good story someday, but at present their reintroduction is plenty.