Probably something stupid. Possibly there was "lighting a vampire on fire in a dark alley because he got too nosy" involved.

Nemo's clucking gets exasperated but it is a more good natured exasperation than most everyone else gets

Also probably Nemo is the one who found out and informed Asch "hey dude the machine just replaced you with a new, douchier model" when Luke started existing so that's a thing

Since I'd assume Nemo hears of the tragic amnesia of Luke fon Fabre through the rich relative grapevine somehow