Pain and pleasure aren't like positive and negative electric charge, or Newton's third law of action and reaction. Sadly, as we know there are a minority of people who undergo chronic pain and/or depression. We wouldn't tell them "That's impossible!" As far we know, its not technically harder biologically to create gradients of bliss or gradients of misery or a mix of pleasure and pain: rather, the difference is how adaptive such states tended to be in the ancestral environment of adaptation.
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