I hope this is the right place for this:
It is the right place.
I'm playing my first game with Happiness. -3 for war with Europeans is a bit stiff, but acceptable.
However, making it -3 for every European country?
You evil warmonger !
How about making it so that every additional European enemy is another -1?
I am not sure.

Basically there would then be
no more real decision "Will I start
another War?"
after you already have a first War going.
It would actually do the contrary:
Once a player has started a first War, he might be more inclined to start even more Wars,
because he know that the difference considering Happiness is just tiny (-1).
"Happiness" should always make the player consider if he starts Wars or not.
(It should not give hidden Players incentives to start a second War after they already started a first War.)
About gameplay:
I wanted "Happiness" to make decisions to start a War tougher.
The decision to start a second War (in parallel) should thus stay as tough as the decision to start the first War.
Your suggestion would simply make large scale "Warmongering" less challenging to manage.
You build up an army and since you already have it you of course steamroll everything that comes in your way - not really caring who you currently attack and conquer.
If you manage "Domestic Demand" and all other positive factors well, you can most likely still handle it. But the more wars you start, the harder it will get though.
The feature was really exactly supposed to prevent these "steamrolling Wars" - where Human Player simply attacks and conquers everything in his way.
(That is actually what you are asking to be removed.)
About immersion:
1. The colonists in e.g. the British Colonists came from all over the world. They were not "purely British".
Some of them were Dutch, some of them were French, some of them were Russian, ...
None of them (would have) liked if the British declared war on their "brothers".
2. The traders in e.g. British Colonist did trade with many other colonies and countries.
When Britain declared War on e.g. France, the trade relations heavily suffered.
This impacted wealth and thus "Happiness" more or less proportional to every new enemy.
3. Having a "two front war" with worsening the chances of winning is not neglectable either.
E.g. in WW2 many Germans did support the War on the East Front but heavily doubted the West Front to be a good thing.
In fact, starting even more fronts would have caused a really severe drop in "Happiness" (Civil Support).
Summary:
In general however it is
simply a matter of taste.
That is why there are configuration settings in "
GlobalDefinesAlt.xml" for almost all features.
Thus players and modders can adjust the settings to their personal taste.
The Define you are looking for (which however does not differentiate between first and second War) should be called something like "UNHAPPINESS_FROM_WARS".
Simply set it
from 3 to 1 if you want to have less Unhappiness from Wars.
You will however have to
start a new game to become effective.