Maybe it's because we're the only two relevant teams in the division the last few years ? You always check up on your rival.
I've said it in other threads, but most of the Niners fans here don't bother me, and I enjoy talking football. I follow other teams as well (not really root for them as much as follow), and I just enjoy football in general. The game is much more fun when there are a bunch of outside factors (kinda like the Niners getting better....it's more fun than just steamrolling everyone like we did in the mid 2000's), instead of "OMG, them Hawks RAWK. We're the best ever. We smash all." all the time. (No, no, I'm not saying that's what this board is, just the exaggerated opposite end of the spectrum).
As for not posting, here's my thing: I know no team is perfect, and I'm probably more critical of my team than most Niners fans are in some ways. Since I know they're not perfect, I don't get butt hurt if a Niners fan comes on here and dares to mention that anything about their team could be better than ours. And hell yes, I've seen it, and it's ridiculous. I've seen some fans get pissed for another team's fan writing something that a Seahawk fan has written, and the Hawks fan jumps all over them.
To other fans: Talk football. Be critical, tout your team, try not to be overly homeristic and troll, and I'm fine with you. You can debate me, correct me, disagree with me, it's all good. I appreciate other viewpoints.