When you love someone your heart beats faster when you are with them/thinking of them. This is because your heart is stimulated by the hormone adrenaline which is produced when you are afraid or stressed or nervous. This hormone is responsible for making your heart beat faster, speeding up your breathing (to give extra oxygen to your muscles) and diverting blood from your digestive system to your muscles (for the same reason). This is the reason why you may also experience “butterflies” in your stomach.
As well as your heart beating faster your pupils become bigger. This is because you want to see this person very clearly and so your pupils become bigger to let in more light when you are looking at them!
When you hate someone the response is very similar to when you love someone and the same areas of the brain are activated. Except this time the adrenaline speeds up your heart due to feelings of hatred. This is why the different emotions can feel very intense and affect your whole body and not just your thoughts,