A quotation from https://en.wikipedia.org/wiki/Self-harm below:
“Those who engage in self-harm face the contradictory reality of harming themselves while at the same time obtaining relief from this act.
It may even be hard for some to actually initiate cutting, but they often do because they know the relief that will follow. For some self-harmers this relief is primarily psychological while for others this feeling of relief comes from the beta endorphins released in the brain. Endorphins are endogenous opioids that are released in response to physical injury, acting as natural painkillers and inducing pleasant feelings, and in response to self-harm would act to reduce tension and emotional distress. Many self-harmers report feeling very little to no pain while self-harming and, for some, deliberate self-harm may become a means of seeking pleasure.”
For pain to carry value it must represent the ground truth and can't be easily ignorable. That makes it carry a very large cost to override which is important since as the examples demonstrate the long-term consequences of an inability to feel pain can be quite severe.
I suppose that ties in to what you said: humans are way too bad a long-term thinking to properly evaluate consequences so we would end up burning ourselves or hacking off a limb to achieve a short-term reward. Or at least enough of us would be that our species would never have made it this far.
Basically if we were to go through life only experiencing pleasure would less pleasure basically be pain if you had no capacity to feel or understand pain.
Plus the whole idea that pleasure and pain are a continuum seems overly simplistic.
My understanding of his answer is if this were the case, we wouldn't always avoid lesser pleasant sensations and may even seek them out to enjoy the rebound to greater pleasure. If burning your skin knocks you from baseline (1 instead of 0 for this example) to 0 (instead of -1), and the relief of stopping the burn kicked you up to 2 as a reward for escaping it, you'll just repeatedly burn your arm. Only negative values ensure you'll never seek them out for the purpose of rebounding to higher values.