How do we understand the fact that the Torah only obligates us to return the lost object of a Jew?
Another place we have this question is in regard to the prohibition of taking interest on a loan, which is only forbidden when lending to a Jew, but not to a non-Jew. The question is, Why?
נראה לי בע”ה that the reason is as follows. (I believe the following is based on the Chofetz Chaim in Ahavas Chessed.) If we think about it, it really makes sense that one should charge interest to someone who is borrowing money. After all, they are receiving a benefit from my money, perhaps investing it in a lucrative business, while I sit at home without the money myself. There is certainly a monetary value to that, which I should be able to charge. Similarly, if someone loses an object, it should rightfully belong to the person who has found it, and not the one who lost it – as my 11th grade rebbe, Rav Yehoshua Kalish used to say, possession is ten-tenths of the law. If you have it, it’s yours. If you lose it, it’s gone!
The Torah, however, tells us a tremendous חידוש. This is that every Jew is considered your brother. A sibling identifies with his brother and would give him back an object he lost, despite the fact that he has every right to keep it. Similarly, a person who loves his brother would never charge him interest on a loan, but do it as a favor, completely altruistically. Hashem expects us to view every other Jew as our brother. This is a special relationship that Hashem desires to exist between the members of His chosen nation. These requirements are extra and beyond what is naturally called for. This is why they only apply to one’s fellow Jew, and not to a non-Jew.