I agree that any emotional state that causes extremes in behavior is a mental condition that needs to be controlled through psychotherapy and perhaps pharmaceutical applications.
Now how to define "extremes in behavior?" I think you have to know the individual pretty well. Essentially, you must define what is "normal" for the individual and contrast that with the extreme. Most of us can do this to ourselves, stepping away from our behavior and saying "Self, you're really getting very weird, aren't you?" This reflection allows a moderating influance.
Then there are those rare individuals who have no capacity to reflect upon their behavior until someone throws a net over them, and drags them off to a funny farm for electro-shock therapy (a la Hinkley, et al. who decided the best way to get the attention of Jodie Foster was to shoot Reagan).
Finally there is another group, of which we all wish we could be members, and who entertains us all with their bold antics: Individuals for which being extreme is their norm (e.g. Elton John, Michael Jackson, Liberachi, Freddy Mercury, Boy George, and IMHO Paris Hilton). The list is long!
I really love FALLING IN love much more than BEING IN love.