I've noticed that some AIs are always mad at me even if I give them gifts or if I am always nice to them. Others get mad at me if I don't give them something, especially right after we first meet. Then some AIs start out being nice to me and I really don't have to give them anything. But the AI's attitudes change throughout the game. I think that you pretty much need to stay in touch with the AIs and try to figure out what they want. I always try to trade with them when I can, particularly with advances. At least keep track of what your neighbor's attitude is - if they are really mad then they might be planning an attack on you!