Not resentment but maybe jealousy.(resentment...jealously....wait is that the same thing? Nevermind) Anyway! Donatello would be jealous because April likes bonehead Casey more than him. It wouldn't have needed to be anything major like challenging Casey to a duel to win April's heart. But maybe he'd squirm whenever in the presence of April and Casey in one of their "love bird moments"
It's sort of like how....well you have a dream about something. But you know the dream has little to zero chance of coming true. But there is that 1 percent possibility that it might and thats enough to make you happy. But when that last percent is lost its a let down.
But since Donatello having a secret crush on April was never truely confirmed this is just my speculation.
I semi relate it to Catherine and Warrick from CSI. Especially this scene which pretty much sums up what I thought Donatello might have felt after he found out Casey and April were going steady.
I've read a few fanfics that run with this idea and i like them. The only thing I don't like is that usually Casey is depicted as a total shell hole which he isn't. But I guess the writer is trying hard to justify Donatello wanting April to choose him over Casey. But the fact is Casey loves April and should be written that way (unless its some sort of AU story i guess)
