*NO!  I must stay
mad!  I HAVE to stay mad!*  But it was hard to do so.

As far as I can tell, they don't have to stay mad.  Futaba and Futana
certainly don't stay mad or excited while transformed all the time.

I think they have to get unusually excited to change and unusually relaxed
to change back, but that when they're in a normal mood, they stay in whatever
form they're in.

(Even that is also subject to plot-device, as when Futaba-chan managed to
stay changed while unconscious because of a very dubious excuse.)