You delay gratification to enjoy future reward. Be frugal now (present sacrifice) to gain compounding investment returns next time (future reward).
The reverse applies. Indulge in bad habits (present reward) and watch yourself descent into misery (future sacrifice).
The worst is knowing that there is no future reward despite the present sacrifice. Promise your children two cupcakes (future reward) if they clean up the toilets (present sacrifice). When they are done, nullify your promise. You have just made them sacrificed their present and future. They will brood on their misfortune and grow up to begrudge sacrifice.