In my current game I'm the King of Ireland. I control all of the Emerald Isle plus a chunk of Scotland. Things are going great for me. I've had several successful kings in a row. My current one has had a highly productive reign only slowed by the unlucky length of time it's taken me to get some fabricated claims on Isle of Man.
I had a mediocre heir who was totally eclipsed by his younger brother. So naturally I arranged for an accident to happen to him. My new heir comes of age and holy crap... 25 Diplomacy! Score! Well time to marry him off. I ring up the Dating Agency and see that there's this beautiful 17 year old with a claim on the Kingdom of Denmark. Her Diplomacy... 29! HOLY ****!!! Soon the wedding bells are ringing and I'm feeling great about the future of my realm.
3 moths later she gets ill and dies. ****!!!! **** **** ****! GODDAMN THIS GAME!!!
Anyway, she was a twin. I marry my heir to her sister who has the same claim on Denmark and has a 21 diplomacy. Still great but after seeing her awesome sister she'll always only be second best.