Commentators agree that Belshazzar's decision was fueled by wine. Under its influence, his pride swelled, and he did something foolish and profane he likely wouldn't have done sober. Scholars like Calvin and Barnes see this as a timeless warning: intoxication can strip away inhibitions, leading to rash actions that dishonor God and invite judgment. It's a powerful illustration of how pride and sensuality are signs of impending ruin.