First the diplomacy requires a lot of work in CIV 6, Some adjustment might be needed or Firaxis could add some options to make diplomacy easier or at a minimum reduce negatives.
At higher difficulties you also seem to have a hidden negative adjuster of the per turn influence so if you don't offset the negative and have a some plus influence amount per turn, relations seems to deteriorate quickly.
Tips:
Let them meet you instead of you meeting them. Easier said than done. (Seems some civs really don't like you being found close to their cities so you start off with say -7 per turn)
Send delegations right away to the civilization you want to be friends with.
Open Borders as soon as possible.
Send trade route as soon as possible.
Give gifts if need or just resource trades.
Watch their agendas and friends agendas is a must to satisfy as the penalty is high if you do not, try to figure out the hidden agenda. Don't waste effort on trying to be friends if you know your going to mess with their agenda. Prepare for war and spend time and effort on other civs
Early AI, will attack if opportunity present itself regardless, specially if they are close to you and have more troops.
Midde AI will mostly denounce first, and they are most likely to denounce if you have lower relations and you do something that warrants it (War, not being true to their agenda)
Cannot do to much about surprise wars, just take them as a man, and make them pay for it.
Last if possible "play the long game", build up positive and let the penalties expire before you rack up more with warmongering. The CIV6 diplomacy is a longer term not instant numbers like civ5, things change overtime based on the modifiers so getting out of negatives take a long time.
The good things if managed is that you can offset warmongering penalty with friends if managed. And the warmongering penalty does go away over time. No CIV5 permanent modifiers. Sure if you conquer a lot it will feel like civ5 difference is that in CIV6 everybody will be mad.
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