People expect to be better off in the future, so a dollar today is worth more to a "poorer" present self.
Böhm-Bawerk did not discover interest; bankers knew it for millennia. But he gave it a psychological and temporal foundation that cut through both the classical "abstinence" theory (waiting is painful) and the Marxist "exploitation" theory (interest is theft). For Böhm-Bawerk, interest is the price of time itself—the premium we pay to bridge the gap between our impatient appetites and the patient structure of production.