Rashi comments that the גר תושב - foreign resident, became wealthy because he associated with Jews, learned from their good deeds and emulated their practices. The Jew who was sold into slavery, however, grew impoverished because he associated with non-jews. But why should this be so? Should the Jewish person not have retained, in the least, his basic Jewish characteristics? Why must he be considered as if he was a heathen and be punished with poverty and worse, slavery?

     Answers the Netziv, R' Naftoli Tzvi Yehudah Berlin zt"l that when we observe society, we know that when a creature dies, it does not recede to the level below it, rather it eventually disintegrates entirely. If an animal dies, it does not descend to the level of a plant - it decomposes. So too, with spiritual matters. When a person on a lofty plane loses his greatness, he crashes way down to a very low level. A non-jew is expected to live a decent and upright life. However, a Jew, being an עובד ה' - servant of Hashem, has a much higher existence. Ifhe destroys this, he plunges to an entirely lower level. This spiritual descent affects his behavior and leads him to poverty and becoming a slave to a גר תושב.