Usually a relationsdifference does a good job explaining this, but on the other hand I've also had succession wars with Aragon over Castile as Portugal when Aragon and Castile hated eachother, so it also isn't a perfect explanation. Perhaps Aragon claimed the throne then, I didn't check.
At least the relationsdifference did make Castile side with me in that case.
This sounded plausible, but I just tested it and at least in the case of the Muskovy and Ryavan succession war over Tver, it made no difference:
- As Muskovy:
- I used the console to boost relations with Tver to +200
- I also allied with them.
- As Ryavan:
- I warned, insulted, rivalled and embargoed Tver, putting relations down to -65.
No change, still succession war.
I am slightly concerned that maybe this particular succession war, which is there from game start, could be different to normal ones.
But I checked through the history files to see if there was anything "forcing" a succession war between these nations, and thus indicating that it doesn't follow normal rules. But there's nothing like that. It simply defines that Tver has a monarch but no heir, and that monarch is of the same dynasty as Muskovy and Ryavan.
So I have to assume that this is a completely normal same-dynasty situation, and what I've tested and demonstrated would happen in the same circumstances throughout the game.
So what causes us to get a peaceful PU opportunity when we share dynasty? I know for certain that it's not only when we are the only nation who has that dynasty.
In one of my games, I was England with House Lancaster. Later, both Portugal and Castile got House Lancaster as well. But it was only me, England, who was getting the peaceful PU with Castile.
Maybe, during the normal flow of the game, the game remembers who "originates" the dynasty? It knew that England has Lancaster first, before it then spread to Castile and Portugal. Therefore England is preferential and no succession war between Portugal and England?