Most AI Civs are pissed at me so when one of them asked for help in a war I was like "Sure let's do this and then we'll be BFF!".
War ended and that Civ is still hostile :( best way to improve my relationship with a Civ? They sure don't care about being helped big time during their wars...
If you go to your relationships tabs, you can see all the different modifiers.
It can happen with these alliance wars, due to a 'warmonger' penalty. You can mitigate this by
1) being on friendly terms with everyone else and declarign friendships/joining an alliance
2) starting wars with a cassus belli.
At the end of the day, the only way to be liked generally is to not join late game wars.
Warmonger penalty scale such that earlier in the game the penalty is light, and the penalty also decays overtime. So doing a surprise war in the Atomic/Information era pretty much means the world will hate you unless you're friends with lots of civs already and they can overlook it.
The penalty itself is a large negative score for declaring a surprise war, plus additional penalties for taking cities in a war.
That said, you can reduce your penalty by liberating cities from another AI that took them. City states are particularly good targets and some AI civs are programmed to gobble them up.
I wish I could put it more succinctly but diplomacy is something you have to learn as you play. It is something you can sometimes ignore, but other times you have to manage if you are in a weak position. I've said this upthread, but unlike older Civ games, each game of Civ 6 is different based on who you play against assuming you play a randomized group of Civs each time and don't pre-set them, which you can do also. Civ6 AI flavors are very distinct, and not cookie cutter like in the past
