It boggles my mind that a fictional person (especially one I've created) can get under my skin, but it happens all the time. Sometimes I wonder if I'm crazy, but then (here's one of the biggest perks of working with a writing partner) I talk to Z and she's just as flustered or excited about a character and I feel slightly more sane.
Sure, you can argue that we're both crazy. And you're probably right. But what the hell are we supposed to do about it? It's too late to stop these characters now!