Wendy Heller's 1993 paper "Neuropsychological Mechanisms of Individual Differences in Emotion, Personality, and Arousal" proposed that emotion could be understood as configurations of activity in certain brain regions. Neuropsychology had until then accounted for each emotion's physical state differently, regarding their relation as too complex for study. It was also different from the emotion theories in other branches of psychology, which sought to explain emotion as a discrete phenomenon not necessarily related to any neurological state.
In Heller's model, there are two dimensions of emotion, valence and arousal. Valence can be regarded as the pleasantness of an emotion, whether it is euphoric or painful. Arousal is the intensity of emotional processing, whether it is memorable and visceral or forgettable and staid. Thus, happiness is positive on both the valence and arousal scales, while depression is low on both. Likewise, calmness has high valence and low arousal, while anxiety has high arousal but low valence. If I had to venture a guess, I'd say being "in love" has high valence and arousal, whereas "loving" somebody has high valence and low arousal. From a strictly observational viewpoint any emotion can be fit into this rubric, so it makes sense as a possibility.
Fortunately for the hard scientists among us, it turns out that all four of these states have corresponding neurological states, and even better, those states can be related to one another. High and low valence correspond to increased activity in the left and right frontal regions, respectively. High activity in the right posterior region signifies high arousal, while low activity there means a low level of arousal. These findings have been confirmed by EEG studies measuring the areas' response to emotional stimuli, and Dr. Heller is currently working with another researcher to get fMRI confirmation of the theory.
Here's an illustration that makes the relationships obvious, stolen wholesale from Neuropsychology, The Neural Bases of Mental Function by Marie T. Banich, 1997. Pretend you're looking down at the top of a head, and the ^ symbol is the nose.
Plus signs mean activity is increased, minus signs mean it's decreased.
Valence
+
__^__ | __^__
/ | \ | / | \
| + | | | | + | |
|---|---| | |---|---|
| | + | | | | - |
A \__|__/ | \__|__/
r Happy | Calm
o |
u + ------------|------------ -
s |
a __^__ | __^__
l / | \ | / | \
| | + | | | | + |
|---|---| | |---|---|
| | + | | | | - |
\__|__/ | \__|__/
Anxious - Depressed