Why can't we charge interest? After all, the very same money could have been invested or put into a bank account, and due to the loan one is losing that added income. Rav Wolbe (Shiurei Chumash) explains that when one's brother is in need of assistance, a person doesn't take into account losses that might result from aiding his brother. Hence, when a fellow Jew needs a loan, one should have the same outlook. We must help him, and this act of chesed should be complete without any thought of remuneration. In contrast, the Torah tells us regarding one who is not our brother; "To a gentile you may charge interest" (Devarim 23, 21 see Ramban).
In this week's parsha we read the prohibition of lending money with interest. "Do not take from him interest, and you shall fear your G-d; and let your brother live with you" (Vayikra 25, 36). Rashi explains that specifically here the Torah adds an exhortation, "and you shall fear your G-d" due to the uniqueness of this prohibition. Not only is there an inclination to charge interest and, therefore, it is difficult to refrain from such practice, one also excuses his actions with, "My money is lying idle by my friend when it could be reaping dividends." Hence, the Torah warns, "And you shall fear your G-d."
Every Jew is a brother or sister. The above pasuk provides two valuable lessons. We must relate to our fellow Jews no differently than how we would relate to our closest relatives, both in material and spiritual matters. What wouldn't one do for a sibling? Some food for thought before the next time we are asked to do a Chesed.
Additionally, Rav Wolbe cites the Ramban on the above mentioned pasuk of this week's parsha. The pasuk concludes, "and let your brother live with you." The Ramban explains this to mean that if indeed you did charge interest, you must return the interest to the borrower, "so that he will be able to live with adequate needs." The Mashgiach concludes, if I am commanded to ensure that my fellow Jew has an adequate material life, how much more so must I ensure that he has the ability to live an adequate spiritual life. If a fellow Jew is having difficulty with any aspect of Yiddishkeit - from its basic precepts to understanding a commentary on the Gemara - we are obligated to offer our assistance.