As you probably know, in the Sims you have no set story, and no set objective. You tell your own stories by creating a Sim (or as in other iterations, a family of Sims, if you so wanted), and then guiding them through life however you see fit. You can take a back seat and let them get on with the finer points of life (usually resulting in chaos!) or you can take a more direct approach, interfering in their lives and bossing them about. The fun comes from experimenting as much or as little as you like, and just seeing what happens.
Unfortunately, that fun takes a lot more work to come by in Sims 3 on 3DS. Obviously, it has been cut down a hell of a lot from the full game, and I’m reliably informed that while, admirably, it’s a totally different game to the standard DS version, it still lacks some basic features. As a result, it suffers from some bizarre design choices that can only point to a game rushed to match the console release date.