This is a general concept post because such questions come up pretty frequently in discussions about epistemology and ontology.
But where DO you find 'love' in the brain?
First, I want to step back from the problem a bit and look at a much simpler question, how does something like addition happen?
This question is interesting because we know that computers do addition, brains do addition, and less well-known, dominoes can also do addition (watch video)!
Where is 'addition' happening in those dominoes? Does any individual domino know anything about addition?
No matter if the addition is taking place in chemical reactions, electronic switches or dominoes the fundamental principles are identical - the process is a cascade of causes and effects and the result is INTERPRETED as addition. Other calculations performed using exactly the same kinds of logic 'circuits' produces a tic-tac-toe game, image interpretation, or could produce language by driving muscles using calculated outputs to muscles.
There is no magic here, what is happening is there is some structure upon which some state is represented.
So the key concepts here are that the structure is important. Without some appropriate structure (be it organic chemistry or dominoes) there will not be the kinds of state changes that map to computational behaviors. You just get 'noise'.
The state changes can be conformational (changes in the shape of the the thing), or based on chemical or electromagnetic potentials. As long as these potentials can 'move' they can be transmitted and affect the states of other structures.
In our brains, neurons and glial cells (along with a cadre of organic molecules that compose them) provide the structure upon which the state changes chemically, bio-mechanically, & bio-electrically.
Some of these neurons are driven by external stimuli that provide our sensory mechanisms (for example, neurons in your eye have pigments that are light sensitive, which triggers those neurons to fire when they sense light, and they can misfire when you apply even slight pressure to your closed eyelid). We only just recently decoded the algorithm (the set of calculations) the neurons in your eye use to encode the visual information from your retina for transmission over the optic nerve, it doesn't just send the data like 'video', it has to be compressed and encoded because there aren't enough nerve fibers going from your eye to your brain to send all the information directly! But now we know for sure, all those neurons are exactly performing a calculation.
Other clusters of neurons in your brain do other types of calculations. A detailed run down of what we know about these regions in the brain would take 1000's of pages but one very interesting region you might want to look up is the Amygdala, which has to do with the processing of memory and emotional reactions.
We can, to some extent, 'measure' a brain in love by looking at activity patterns in the neurons. This is a very gross, or high-level, view and it doesn't tell us much in detail at this point in time but we do indeed find neuronal correlates to love in the brain. And using ever improving tools we can study the brain in love:
Helen Fisher: The brain in love
Love's all in the brain: fMRI study shows strong, lateralized reward, not sex, drive
Neuroimaging of love: fMRI meta-analysis evidence toward new perspectives in sexual medicine
How Two Brain Areas Interact to Trigger Divergent Emotional Behaviors
Rebooting The Cosmos: Is the Universe The Ultimate Computer?