I seem to find the AI's willingness to make peace to be highly dependent on their relationship with you.

I have actually been at war with someone who was "indifferent" due to state gift, same gov type, respected leaders, etc. And they would make white peace at any time. Demanding things was, as usual, expensive if I wanted anything, but peace itself was no problem.

The repeated peace/war/peace/war is probably because you have decent relations with them so they accept peace, but they are hardwired to obtain objectives one of which you own and you share a border.