Recently I have been attempting to do a Danish playthrough in vanilla EU4, and though I am able to diplo-annex Holstein, keep Norway's liberty desire at 0% and even expand eastward and take a few provinces from the Livonian Order, Sweden always seemed like a bucking bronco that wants to viciously tear away from their overlord at the first possible chance they get. My ultimate goal is to form Scandinavia, so I'm wondering what would be the best strategy to keep Sweden from declaring a war for independence just long enough for me to properly integrate them.
I suggest you read the EU4 wiki page if you want detailed information on what influences a subject nation's liberty desire (LD). What I consider the "easy to manage" factors would be as follows. Note that at any time you can go in your subject's section and hover your mouse over the LD number to get the more information of the current LD value.
- Make sure your army is stronger than Sweeden's. They can get up to +75% LD if they ahve 100% of your army size (scales linearly).
- Don't feed them any province. The highest development they have, the more their LD increases.
- Make sure your Diplomatic Reputation is high. The lowest it is, the more LD you incur in your subjects. After annexing a subject, you suffer a -3 DR for a few years (10 or 15 if I'm not mistaken). This can hurt your relations with your subjects.
- Keep high relations with Sweeden (always +200 if possible). Again, after annexing a subject, you get a penalty to the relations with your subjects (-30 or -50, I can't check right now).
I would suggest you buy the Common Sense DLC if you can/want. Among other things, it gives you more options when dealing with subjects (such as placating the subject's local ruler, reducing their LD by 10% at the cost of 20 prestige).
The key factor is fast expansion. I find the best way to do this is to get the "Danish Trade" mission at the start of the game and take Neva + Novgorod and Ingermanland before Muscovy takes them. These are valuable provinces and they allow you to get the trade flowing towards you from the East early on, which in turn enables you to keep an army that is large enough to keep Sweden in check.
Once you have established yourself in Russia, make sure you get allied to Poland (hoping they have the personal union with Lituania) and slowly eat up the Russians to maintain your dominance over the Swedes. Combine this with some campaigns against Livonia and the Swedes will soon lose their desire to mess with you.
Important to keep in mind here is that you HAVE to let your Junior partners and allies do most of the fighting for you. If you let yourself get draw into big battles and lose a lot of men you run a substantial risk of dropping your power below what you need, in which case the Swedes will immediately withdraw their forces to their home turf and leave you to be beaten up on your own.
The events built into the game will still cause the liberty desire to peak over 50% incidentally, but this way you will certainly succeed in forming Scandinavia.