I think the problem is the lack of sense of the "tall poppy" effect (in short, the system who makes AI factions improve their standing with little facions and worsen it with larger factions, every turn).

I'm with those who accuse it of being irrealistic: if your neighbour is thrice your size, army and economy, how in the Nine Hells would you have it as an enemy instead of having it as a trusted ally (or liege lord)?

There's no way in real world you'd attack him. And this policy is what in history all the nations did.

No need to speak about it further as it has been debated to death, but such is the "tall poppy" system (but Diplomacy Mod removes it).