When you fulfill Sim's dream, you get life happiness points. You can check the number of points you have in a proper fold (chest icon). In the same place there are prizes you have chosen for your points . When you gather a proper number of points, you can buy yourself a certain "burner". In case of every wish you can check how many points you can get for achieving it. Of course you get much more points for achieving lifetime wish. List of prizes is very long , and their cost depends of the strength of its influence. You can make Sim eat less often, his or her friendship will last longer or you can change Sim's traits or even life wishes.