William and Uberta were both in arranged marriages that didn't turn out so wellMaybe 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.
Derek and Odette planned their whole relationship from the beginning.After the initial meeting, the two made up and decided that they liked each other, and that getting twice as many subjects and wealth when they grew up was a sweet arrangement. However, they pretended to fight and not get along for the next decade or so to troll William and Uberta. They were in a serious relationship and intended to get married since their teenaged years, and planned the whole "Is beauty all that matters to you?" incident ahead of time for fun. The next step in their plan would have been a big reconciliation, but Rothbart kidnapping Odette screwed that up. Once his lifelong friend and lover was taken away, the gloves came off and Derek stopped pretending.
Adult Odette is an imposter.The actual Princess Odette that Derek spent his summers with died of illness or an accident, so King William adopted a niece or young cousin or something and passed her off as the real thing the next summer. Derek and the Imposter Odette already knew and liked each other, but were unable to be together because he'd been betrothed to the real Odette all his life. When they replaced Odette with the girl he liked, Derek went along with the operation enthusiastically. However, he got tongue-tied when the Imposter Odette asked him about his feelings for her, and gave her the wrong idea that she was just a pretty Replacement Goldfish for the real Odette. After Derek went to such lengths to free Imposter Odette from the spell, however, she realized that it really was her that he loved all along and not the real Odette, and the two never told a soul who she really was.