The prohibition of “Ribis”, interest, is unique. The Torah even directly connects this Law to Yetziat MItzraim (Vayikra 25:37-8). Not just collecting, but even borrowing with interest is illegal. Unlike theft, which is asur from a Nochri, interest is permitted. There is an emphasis of “Achdus” in this Mitzva
(לו). “וכי ימןך אחיך” (לה)…”וחי אחיך עמך”
Would one charge interest to his (biological) brother in times of need? With this Mitzva the Torah instills feelings and obligations to all of Klal Yisrael. Indeed we are all brethren.
Kli Yakar adds another insight. Parnasa comes from Hashem. Most people struggle and are not at all confident that they will be financially successful. This is surely true in an agricultural society, prevalent in days of yore. By contrast, the money lender has no worries. His profit is guaranteed, regardless of his efforts. He may become proud and forget that it is a façade, covering the truth of Divine distribution of prosperity. The Mitzvah of Ribis comes to bolster our Bitachon. (This message is especially befitting Lag B’omer)
Shabbat Shalom