Rather you are giving or receiving, what step makes you most aroused? Is it the preparation, the waiting before the nozzle is inserted, the actual feeling of the flow, is it holding the solution, or the actual relief of releasing ?
For myself, I definitely get a butterfly feeling preparing the bag, lubing , and getting into a semi comfortable position for what is about to happen. The arousal for me stems from the actual feeling of being filled , the moment before or post cramps.