Is it commitment or a lack of other options at this point? Who else is Ozaki going to turn to than the guy who's seen the proof positive of what's going on? Of course at first both chose each other over anyone else but a part of that's their position. By now Ozaki's got no one else and Seishin's recently acquired Sunako and is turning to her about as frequently. Part of Seishin's angst is that there IS the Sunako factor.

They've been back together for a few years now so that's given them some time to rekindle and also develop a new relationship. I wouldn't be so quick to write off the propinquity effect but like you said that can't be everything. Tamo's definitely out of that loop as another pillar and Ishida who was professionally involved was just kind of a third wheel Toshio manipulated and whom Seishin was exasperated with Toshio over. There's definitely a balance between them that isn't struck with anyone else, especially from Seishin's side. Toshio will talk about Seishin being naive and idealistic to Ishida but Seishin will go directly to Ozaki with his suppositions that Ozaki's just being a control freak. Seishin at least clearly sees their duties as something they discuss and share as individuals separate from just being a duty. He told Ozaki off not out of his duty as a pillar but because of how he felt about Ozaki individually I think; his exasperation with him was the same, not "you're endangering the lives of our village here!" but "Toshio, my friend, you're being a dick." The "lives of the village" aspect is more Ozaki's angle.