AUTHOR GOES ON A MENTALLY DERRANGED RAMBLE BELOW??
I think Seimei is overly emotional all the time because he’s tapped into his aura and magic ability so much. Being really naturally gifted as the most powerful diviner in the country (and used by the Bafuku for omens and foretelling) it’s also his achilles heel. Instead of explaining his feelings verbally, he rather explode. Such as not telling Douman how he felt about him when they were kids, and later on Douman being in a relationship with his sister. Instead Seimei begrudgingly bottled it up and/or became furious, sorrowful and heartbroken. Like even in gintama canon he mentions he can do all the spells in the world but he can’t repair a friendship with an ex-friend.
Whereas Douman knows and accepts he’s not the strongest diviner especially compared to Seimei (he admits this in canon too later on), and doesn’t deny it. Magic is still an important part of his heritage as an onmyoji but unlike Seimei who absolutely lives for it and loves studying it all day at home, Douman seems more interested in pursuits of an artistically and theaterical nature.
They’re both crafting artists in their own rights and complement each other with their differences yet similarities.
End of mentally unstable rant.