Relationships at the start realistic.

[Albo 16]

I tend to hop into scenarios like Modern World as the UK and off the bat get insulted by all of the nations who in reality should be my friends. What I'm trying to say is that the reason to why nations change their diplomatic relation to you seems to just be pure RNG. One minute I'm an ally who fought with them in countless battles the next we have cut diplomatic contact.

[Shehab666 43]

This game isn't meant for realism but I agree on one point:

I played as Castille on 1440 and what happened was I allied with Portugal to take out Aragon (they had some Portuguese lands) and after the peace deal Portugal declared war on me!

The AI needs to follow alliances, friendly and rival relations.