The Lewis acid(LA)-catalyzed Diels-Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled-cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I-2<SnCl4<TiCl4<ZnCl2<BF3<AlCl3. Emerging from our activation strain and Kohn-Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels-Alder reaction by a diminished Pauli repulsion between the pi-electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels-Alder reaction by enhancing the donor-acceptor [HOMOdiene-LUMOdienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.