Some have suggested that a bit of pain might be necessary to make all the pleasure we're supposed to get realizable.... If the only way to make people happy is to improve their lives....

Taken to a literal extreme (I don't know if that's your intent), the idea that pain is necessary for pleasure violates the Generalized Anti-Zombie Principle, or something like it. If pleasure without contrast palls, there's some neurological reason for this, one that we could work around in wireheading if we really wanted to. I think the most you can plausibly say is that for humanlike architectures, memories of suffering (not necessarily true ones) are necessary to appreciate pleasures more complex than heroin.

Personally, though, I think that there are already plenty of humans who, through genetics and/or introspective self-modification, can be perfectly happy without improving conditions.