Yes certainly this gets into a branch of philosophy, about what all our emotions and feelings are about. All of them begin in the brain and begin with impulses we receive. So we see someone that we care about. Our eyes dilate, our heart beats faster, we get a little sweaty, our stomach gets a little wiggly. These are all signals sent to our body from our brain, because of releases of hormones.
And most of it has to do with survival of our species, i.e. sex. Our body hormones flush our system when we see someone we are attracted to, to make us more 'in the mood' and also to put out endorphins to make our partner more 'in the mood'.
Jealousy is similar - if we see a threat to our relationship with the person we love, we instinctively begin to take steps to make sure that threat is addressed ... many people say they can't "help" being jealous. It is just something their body does naturally.
