Mh okay i can try a top 10 list. I play mostly on immortal difficulty with huts and events, but also on deity but deity difficulty itches and scratches and is not as much fun.
1. It does not matter if you open borders with someone i think. That does not affect diplo, only when they ask you to cancel your trade deals ( including open borders)
you get an penalty with the Civ you canceled the deal with. Then you have to kinda like choose.
2. If you settle close to an enemy you will get border tensions which will result in negative diplo. If you join in on an war as a 3th member you get bonus with your ally too.
Also when A.I. runs out of free land to settle, they tend to search for new "Lebensraum für das Volk", they want your land.
3. Check frequently the Foreign Advisor (F4) glance screen, if some Civ is the worst enemy of another Civ you get negative diplo if you trade techs and resources or gold with them.
For example Montezuma is worst enemy of Pacal, when you make trade deals with Montezuma that will hurt your relations with Pacal.
4. You can improve your relations with other civs when sharing a religion (obiously) and also choosing their favourite civic. You can check the 2nd advice also in
Foreign Advisor (F4) info screen.
5. If you trade resources to another Civ. For example bananas for wine, it will give bonus relation after a couple of turns. dont know the exact number but i guess like 30 or even more. Also if you gift them techs or gold will help, and also make good trades to them. For example you trade Civil Service (1352) beaker for Priesthood (110 beaker).
6. Especially early game it is hard to improve relations. No religion, no alpha, no civics, no currency. Usually only way to impriove relations is to settle a new city and gift it,
just make sure it is a bad city, for example in the desert or tundra, and will do more harm than good.
7. The most A.I. with agrressive trait, for example Montezuma or Shaka will produce a lot of troops and like to attack. But not only these guys for example also Julius Caesar and Napoleon. Others like Ghandi or Mansa Musa will rather tech a lot and be peaceful, Protective leaders (sitting Bull e.g.) will spam a lot of troops and stack them in their cities. You need to read the guide i posed to understand how they behave.
8.Stay out of religions early game if you are unsure about your diplo game. Nothing ruins your relation faster than be in buddhism while all others are in hinduism.
9. You can improve most of your relations with a small amount of gifted gold as soon as you get currency. Like 20-40 gold.
10. Do you have "Bug&Bull" Mod installed? The red fists shows that someone is plotting. Usually but not always they attack their worst enemy. You can decrease the chance of being attacked when being pleased with most civs, and when friendly nearly no other civ will attack you. Julius Caesar and some other heavy warmongers are the exception, they also can plot an attack at friendly. If they are cautios with you, you are always in danger, ofc even more when they hate you.
11. If you are isolated with an A.I. (continents) for example.
Here i give the 3 most common ways i use to avoid a DoW (declaration of war) on me, when i see red fist:
a) Build a scout/chariot. Open borders with them and take a look, if they move their troops towards you. Then it is likely they gonna attack you.
b) As soon as you see the red fist appear and you think you might be the target, beg something minor. 20 gold, 4 gold/turn, a tech you nearly finished etc. Then you will get a 10 turn peace treaty where they can t attack you. often they also just stop to plot. Red fists disappears.
You can only beg like once in 30-40 turns and only if they are pelased or friendly with you. So be sure to save your begs when it counts.
c) Bribe another civ into war with the A.I. that wants to attack you. Then of course they will not go into war with you, because they are already in war.