Reading from his AI file, Sitting Bull is quite the isolationist. He isn't the most agressive, but he does make frequent demands, witch can sour relationships. On the pluss side he is very unlikly to declare war if he is not your neighbour, and he also values friendship more than most.
Pericles seems by looking at his data to be a moderate warrior, the emphasis here is placed on wonder constructing and tech trading. He is a good neighbour. Why he refused to talk to you seems like a bug. I thought they only refused to talk at the begining of wars. Perhaps he was the vassal of someone else?
KK