If we assume pain and pleasure are opposite and exist on a continuum, there must be some sort of neutral point between them. Things can be neither pain nor pleasure. The average of the possible experiences is neutral.
If we only experience neutral things and pleasure, then in fact we have moved the baseline. The average is now halfway between what "was" neutral and the most pleasurable thing we've experienced. Pain is always defined as the low end of the scale.
If we connect this to real-world pain and pleasure, we get into biology. Pain represents harmful things for an organism whereas pleasure represents good things. If all you had was "good" and "really good", really good would be the new goal, the new pleasure. Organisms would evolve to feel discontent or pain when they are not gaining an average amount of "good".
In other words: When you have less pain, the baseline moves, make what was pleasurable before become less pleasurable or even painful. Similarly, lots of pleasure broadens the spectrum and also increases the baseline (beyond the again biological effects regarding novelty).