This reminds me of a point I've heard. Basically, the primal act of fleeing a predator is stressful, but that stress is a healthy response, and in some sense they get it out of their systems and therefore don't form generalized anxiety. Supposedly, humans only started getting generalized anxiety after we no longer feared predators. On the one hand, this rings true for me. On the other, it seems like an ad hoc explanation that just sounds really good.