Why study emotions objectively?
What are emotions? Well, according to a book I just read, Crucial Conversations, they are the result of the stories we tell ourselves.
Those stories being subjective, the emotions, too, are subjective--and subject to change. But don't try to tell someone who felt bitter disappointment that it wasn't real, just because the story upon which the feeling was based wasn't.
Wasn't real, that, is.
That is, just because an emotion has been transformed by a transformation of the story that generated it (if not vanished completely), doesn't mean that the emotion as felt wasn't a real experience.
This story itself may be conscious or what psychologists might call, sub-conscious, but it is the conscious experience of the feeling that we can say is real and objective, in addition to being subjective.
Emotions are real to the observer, so there can be an objective measurement of them, not subject to being subjective. It's even possible that intensity, long felt to be a critical component of emotion (pun intended), is a subjective criteria itself.