Have you ever wondered why a self-made not so delicious food tastes tastier than a meal from the restaurant? Or why you like a custom made shoe color design no matter how odd the combination may be, if you were the selector of those colors? The answer lies in the power of Self Creation, Dan Airely call it as an Ikea Effect. We as humans have an imbedded mechanic to give more value to things that we have created, or have invested time in it. Perfection weighs less, and no matter how imperfect your work is you will prefer it, maybe not explicitly but deep down internally, if you get a chance to up vote among a few, you will favor yours.
Working inside a game, where you develop your own virtual world, you will find your guild and your possessions to be dearer to you than to any other outsider. Games use this mechanic very carefully, hence harness a longer engagement from the players, as leaving what you have created, with your precious time is not easy psychologically.
Another example will be the hours you spent on your farm, in FarmVille, growing it. Even if you have a sudden realization of its no realistic value, but it is eating more and more of your time as your farm grows larger and larger you are unable to stop playing it, because you value your creation and hence want to enjoy it and feel complacent with it.
Using the psychology of Self-creation as a mechanic in your gamification design, can be a very powerful ingredient for increasing the engagement. The design should be such that the players feel a sense of self-creation among the activities they are doing. May be the design should provide hints, but to the player it should look like that the actual work was done by the player and not guided by the system where the player is just a viewer. The more the player is able to create the more valuable the creation will become and hence the intrinsic motivation of feeling proud will work for a long term engagement.