It's true in the sense that "good" is a subjective term which is not consistent with any baseline, other than personal experience.
As such, if yeu experience "good" for too long a time, that becomes the new baseline of "normal" or bland/mediocre/average. We tend to strive to attain more than we currently have, not because we "always want more", but because we aren't satisfied with "good enough" for long periods of time, since it gradually drifts into an average category.
No matter how intense or amazing yeur life may be, there's always "more" or stuff yeu could do to further that. I wouldn't be surprised if this's a key issue with why so many celebrities end up doing hard drugs... after yeu're rich and famous, then whot do yeu do to improve yeur lifestyle? All yeu can really do is retreat into a fantasy world at that point. Once yeur life is so extreme for so long that yeu consider that 'normal' and boring... there's really just not many places left to go.
Now, that being said, some are alright with whot they have, never changing... those generally are individuals with brain damage, however, or a severe lack of memory, to the point they don't remember whot they have so it's new every time they see it.
Even the strictest J, will eventually want more money, a better house, a nicer car, a faster computer, new games, new opponents, new challenges, new 'stuff'. Any, or all, of these can apply, regardless of how static one may desire to be.
While most people are easily swayed and just want "more", those who are content with whot they have, are still affected, but in a different way. If yeu're alright with the stuff yeu have... then yeu want something else. Spiritual enlightenment. To know more. To understand more.
These aren't even the only options either... for example, I don't care about how good something looks on a computer, as long as it's fun... yet I'm in a 3d graphics course specializing in game animation. Go figure XD The point is though, I was, and STILL am quite content with the previous games from a decade ago, that still entertain me. I just bought serious sam (first encounter) and the original aliens vs predator today (they were dirt cheap on steam XD ). And yet... I wanted 'more'. Not graphics, but I end up wanting a better computer anyway. Why? Well I learned earlier this week that my computer, although fast for games, flat out doesn't have the processor power to do massive render queues... doing hundreds of frames at a time, or thousands... it helps to have it go faster XD
I also eventually will tire of the games I have, the programs I use I'll want updated stuff, I'll want something to replace the things I've gotten tired of... and in so doing, I'll be eventually find myself wanting a faster video card, more ram, other computery bits. That kind of stuff. It's a pretty good parallel of an analogy, honestly. Even if yeu have everything yeu could ever dream of right NOW... eventually yeu'll grow tired of whot yeu have, and someone else will have dreamed of something more interesting than yeu have right now... and yeu're going to want it. It's an endless cycle.
But yeah, I don't think it's that we're never satisfied. I think it's moreso that we're satisfied but only temporarily, until we get bored, or our standard of quality increases over time as we improve our lifestyle over time. On the surface, it looks very similar, but the mechanics behind it are quite different.