Dan Ariely, in his book “the upside of irrationality” talks about case of pleasure. He argues we as human beings are capable to adapt to pleasure, and hence we do not enjoy subsequent pleasure as much as we enjoyed at the first touch point. As you can see in the figure above, the pleasure or happiness declines considerably over time, if you purchase all the goods at one point (blue dashed curve). On the other hand if you purchase in parts, then by the time you get adaptable to pleasure from the previous purchase, the next purchase gives a kick to your happiness / pleasure experience (red curve), and hence in nutshell the area covered or the happiness/pleasure experienced over time is much more than compared to the one time shopping.
Relating this to Gamification, an engaged user who is rapidly coming back to your website or using your product, given that your gamification design runs on the trap of PBL’s, then following the concept of the hedonic treadmill (supposed tendency of humans to quickly return to a relatively stable level of happiness despite major positive or negative events or life changes; wiki) the player will lose the interest. There will come a point when reaching a new level will not give the same happiness that gave on the previous one, some what like diminishing returns.
For e.g. you run a website, and have incorporated a PBL platform to motivate the user for sharing and engaging with your content. As the user moves up your leaderboard and gets more and more surfeit with your content his/her engagement will decline. Think that this player is now the top player on your site, there is nothing more to accomplish, than unlocking a few more levels, he/she will feel bored, and look for other places to derive pleasure.
Rather than giving points in a non surprising ladder fashion, if the design is made such that the elements of curiosity and surprise are embedded, which keeps the player thinking and guessing what surprise is stored next; will keep the players’ pleasure curve high, thereby ensuring that the player stays much longer engaged with your system. Give the player few accomplishments, then wait till the engagement gets low, then push a booster package to kick back the pleasure curve.