Correlating brain activity to thoughts is not the same as being able to distill love or emotional experience down to objective understanding. The difference is spiritual experience.
Oxytocin is a part of how people experience love, but it will never be possible to objectively assess whether someone is experiencing love by measuring it or any other physical quantity.
We can measure the wavelength of light and track how it stimulates cone cells and the brain, but we will never be able to measure the spiritual experience of color.
It is science that will always be chasing the 'gaps' in measuring spiritual experience. No matter how closely we can measure ourselves physically, the actual spiritual experience will always transcend it.
Trying even to describe spirituality at all is difficult because it's an inherently nebulous thing. It can only be known, never proven.