[capnq]

Originally Posted by Mabus

Whenever I make a treaty with an AI player, they immediately began growing angry. Peaceful AIs like the EEE take longer, but within a few turns they will turn murderous and declare war--it's inevitable, unless I go ahead and break it off first. [...]
Maybe I'm missing something obvious, but I am very confused. Is it a bug? Something related to my play style?

In each race's racename_AI_Settings.txt, there's an entry called "Get Angry Over Allied Colonizable Planets" which is usually set to TRUE. You might try changing that to FALSE, at least for the friendlier races. (I've never understood the logic of that defaulting to TRUE.)

Generally, the AI's mood won't improve as long as they can see you in systems that they've claimed. Gifts and favorable trades are about the only interactions they'll put up with, and they often won't accept those, either.