Maybe that's because our sensory systems can't really feel states, only differences between states. Hot only exists in comparison to our memory of cold. Without that comparison, there is simply numbness.

If that's a general principle, it implies that happiness can only be felt as an improvement over the past. That would also explain why sadness is necessary for us to be able to feel happiness, and why we're attracted to tragic art. Someone who is the same level of happy all the time will become numb to it, but constantly becoming happier is unsustainable in the same way as a drug addiction.