Sounds plausible, Martin Alvito, but the example with Korea I posted happened on Immortal, and Korea was stronger than me according to my military advisor.
You'd think that military strength plays a role, and I certainly think it still does, but it looks like a civ stronger than you can still be afraid of you in much the same way as a civ that is weaker than you can still go to war with you.
In the XML files civs get number for what's called 'Major Civ Approach'. The possible approaches are Neutral, Friendly, Afraid, Guarded, Hostile and War. A civilization always picks one of these stances towards you. Sejong's number for 'Afraid' is a 7. His other numbers are lower, so 'Afraid' is a stance he'll quite easily take.
All other civs have lower numbers for 'Afraid', except Montezuma, he's got a 7 as well. But Montezuma's got an even higher modifier for 'War'. It's a bit strange that an aggresive civ has such a high number for 'Afraid'; it seems out of character. All other warmonger civs have a very low number for 'Afraid'.
I'm not saying that these numbers are the only factors, but they will of course play a big role, they're designed to do that.