This isn't the most groundbreaking realization, and I'm probably not the first person to think of this, but I still thought I'd share it.
Basically, all pork sausage is ground from butt, and butt, being a heavily used muscle, is full of collagen/connective tissue. Connective tissue requires long wet cooking methods to dissolve, and, until dissolved, collagen can be tough and sinewy. I slow cook all my pork butt products, but for some reason, I've never thought to slow cook sausage. Since I have, the texture is far more succulent and far less sinewy.
Presently, I've only been simmering it, but I might try some kind of slow roast next.