naturally warlike people will eventually attack their friends. Look at history....when warlike nations run out of people to make war against, they break treaties/alliances.
relations gradually slipping to -80 is the reaction of an aggressive and warlike or greedy people who, upon taking the 2 YEARS of no attempts to improve relations that it takes to go from 0 to -80, have finally decided "you know what? maybe they're not worth being friends with."
As to why go to the peace talks? the "success" result is usually a massive boost in relations...often 50 or more, and in my games, usually results in an instant cessation of hostilities (again, when it succeeds). When that happens, I have 2 years (8 quadrums at -10 each, and it needs to hit -75 to go hostile) to do SOMETHING nice for that faction before they'll attack me again...attacking a pirate outpost, fulfilling a trade request, or even periodically trading with their caravans will drastically extend that time.
In my games i've started playing with double factions (2 of each type) just so that i'm not friendly/allied with everyone except the pirates by the time 6 seasons go by. 8 decaying relationships instead of 4 actually means i'll consider those trade requests or pirate bases even if the reward is only marginally useful (11 prosthetic arms is still a "no" though, sorry)