Rivalries always involve history and both teams being good. Take, for example, the first decade of the 21st Century. The 49ers were barely an afterthought in our Universe. While we were winning the division nearly every year, the 49ers stunk. It wasn't until the 49ers were good again, then the Seahawks started to get good again did the real rivalry begin and did the mutual fan bases begin hating each other. Once the Rams get good again, I imagine that old rivalry will heat up again as well.
True rivalries can't exist with only one of the two teams being good. Both need to be good at the same time for it to be felt mutually.
The Seahawks "rivals" in the 80's were the Raiders and Broncos. Yet ask any longtime Raiders and Broncos fans if they hated the Seahawks. Not at all. We were almost always an after thought. Outside of the success in the early and late 80's, the Seahawks rivalries were never reciprocated with a mutual feeling of disdain.