The Swan Princess/WMG

William and Uberta were both in arranged marriages that didn't turn out so well

Maybe they were in love with each other, before marriages were arranged with other people, maybe they just both ended up unhappy in their marriages, but whatever the reason, William and Uberta were in marriages arranged with other people, and the marriages weren't the happiest ones. This is why, when they had kids, they were willing to spend essentially every summer trying to get their children to get along, instead of just shoving them at each other at the age of majority, and telling them that they were engaged- they wanted their children a chance to fall in love with each other.

Of course, they probably didn't foresee how much they would fight with each other, or anything, but they at least wanted to give their children a chance.