@Reademup I sorta agree too. I think Osiris also likes Set because he understands him and maybe feels a bit special for being one of the few if only one that can ‘tame’ Set.
With Set improving himself Osiris feels like the Set he knows is disappearing, which could ultimately lead to his downfall. Since Osiris was mainly chosen to rule since Set’s personality wouldn’t suit ruling, once Set was deemed a better man, not to mention his contributions to the war, the people will naturally support the stronger one out of the two.
Osiris could be paranoid of him being overthrown by Set since once again, Set’s developing into someone Osiris isn’t familiar with.