Hashem constantly challenges each indivdual according to his or her own level. We are continually placed in situations in which we must choose between good and evil - or between the good we are capable of achieving and the evil to which we are liable to succumb. If we pass the challenge we condition ourselves for good, and we are ready to face a slighly harder challenge. If we fail, we slip ever so slightly - and Satan will tempt us with commensurately worse temptations.
(Rabbi Eliyahu Dessler)