The fact that at the beginnig everyone is neutral means that everyone has the same potential to love/hate you and each of everyone.
(notwithstanding cultural aversion and "historical" hate, but the malus/bonus are low, you can easily outweight them with proper care)
The easiest way to win the diplomatic game, is to paradoxically dont take the initiative:
- in the first turns, dont sign anything that isnt crucial to your own existence, let the AI work each other out, stay neutral,
anger noone, please noone, discover factions as fast as possible (stay below the radar)
- after a time, analyze the diplomatic network of the factions you have encountered, work out their foes and their friends,
work out who you want as friends and who you dont care as foes (plan ahead)
- pick some and dive in
As soon as you start having a diplomatic agreement with someone, it opens him up to further agreements. But the most important thing is that it opens
their friends to further agreements.
As soon as you sign your first deal, it has an impact on the network of the faction you signed with,
which is the whole world.
A faction that is open to you, will open their friends to you too: the first faction being the "door" to the rest.
Once you have signed every diplomatic agreements you could with the first faction, their friends will have a relationship bonus with you.
At least one of them will want to sign something with you, and this will work toward more and more agreements with the whole group. Good job, you just made everyone in this bubble your pals and will probably have a very stable peace/commerce/alliance with them, if you wish to, and if they dont go full retard on you.
The only downside is their enemies, which will become hostile to you if you dont have enough diplomatic agreements to outweight the hate they feel for the fact you have treaties with their enemies.
Most of the time, except if those "to be" hostile factions are dangerous to you, dont sign anything with them and break anything you had, so that you dont have the "-X treaty with Y" with the guys you made friends with in the first place and endanger the network you created.
In the end you have a cascade of positive bonus to those you made your friends, because they all have multiple "+X treaty with Y" with you, which easily puts you in the 50/100/200+ relationship value.
From there on you can choose to do 2 things :
- use the positive relationship to become even closer, to the point where you will certainly go to war at their side and have even more relationship bonus.
- use the positive relationship to paradoxically try to become friends with neutral or enemies factions to them, where the positive relationship will outweight the negative events this will lead to, this way you can have friends out of one-sided blocks
bonus option:
Of course, since the world is not stable and random events changes everything at every turns, you will have to adapt to some situations, and sometimes you will lose control. Also as much as the "+x treaty with Y" snowball effect can go in your favour, the "-x treaty with Y" will also create situation where you create a block of enemy because you have made friends with someone's enemies, whose friends will also hate you for it.
The first few friends block you will choose are the easiest to manipulate, since you began with a neutral relationship to all.
The longer your campaign, the less control you will have other the diplomacy, because you basically reap what you sow in the beginning.
After a time you will reach a point of no return with most of the nations, and to change this you will either have to completely modify your own network (break deals, go to war against former friends, give large sum of gold) or will have to conquer the nay-sayers and eliminate/vassalize them
Of course, you will sometimes have to change your plans, according to your military or financial situation (expansion and trade are necessary), but you can use the diplomatic situation as your basis for what you should do and what you should not.
I dont know how the exact relationship modification value is chosen: I've had many "at war with" = -1 or "battle support" = +40 or some other strange things. This is hard to predict thus you have to be extra cautious if you want to keep your friends as friends, and your enemies as enemies.
Basically, hover on the diplomatic map a lot, read the tooltips, and use the information as a spearhead for what are your goals : either role-play goals or down to the ground military objectives.