He probably has hidden negative modifiers. He might secretly covet your lands or be angry that you built some Wonders he had started to work on.
Have you been denounced by others? If he likes you more than them, this may well have played a role.
Keep an eye on him, he should open his game more eventually and you'll see the negative modifiers.
It could be many things. It's hard to tell without knowing where you stand with the others and what you've done. Some AI are devious/lying, and prone to backstabbing. I don't have Polynesia, but Bismarck is one of those two-faced AI, and Maria Theresa (apparently Boudicca and Dido are some of the worst but they've been relatively loyal in most of my games). Pocatello is another. He pretended to be friendly for most of the game, we had several bright green modifiers and nothing else, then after he denounced me and became hostile, I discovered he coveted my lands, was angry over Wonders and about competition over the same city-states.
Like someone suggested, look for signs he's not liked by others AI or bribe him to attack someone he has a DoF with, in short try to get him chain denounced for backstabbing, and keep making friends yourself. It's often a good way to deal with AI like that, or they'll manage to get you chain denounced.
i'm glad to see another fellow confused ciV player bask in the forum knowledge
