Is pleasure a state of being, or the passage from one state to another?

When I was child, I had this great epiphany. It was a hot day and I remember walking into an air conditioned store. I felt a great sense of relief as I entered. The air conditioning was not very satisfactory and after a while I started to feel uncomfortable. How could I feel good again? What if I walked outside and came in again? It worked! I tried this two or three more times to verify my observation. Being just about as nerdy as a child as I am now, I concluded that at least some pleasure results from the cessation of unpleasantness.

Scientists have identified a pleasure center in the brain of humans and other mammals and relates to the release of dopamine. The pleasure center is known to be stimulated by drugs, food and sex. However, the effect wears off. Hence my question. Must there be a change in state for the pleasure center to respond? It would seem to be the case. Otherwise someone would have come up with a way of continually triggering the center, leading to a permanent feeling of euphoria.