We could be in perpetual bliss, but then we'd be passive. Members of the community that raise their 'discomfort' bar to whatever the current environment provide

We could be in perpetual bliss, but then we'd be passive. Members of the community that raise their 'discomfort' bar to whatever the current environment provides, become more proactive and thus increase their calories, social situation, ultimately their reproductive fitness. We're in pain and fear because nature tuned us to ride this thin edge - unhappy but not too unhappy, in pain but not (usually) in debilitating pain. So we get off our butts and find a mate/home/food/safety in greater abundance (than our peers).