A promising young man should go into politics so that he can go on promising for the rest of his life.

Byrne’s quip plays on the double meaning of “promising.” A “promising young man” is conventionally someone with evident talent and a bright future; in politics, however, “promising” also means making pledges to voters. The joke suggests that political life rewards perpetual pledge-making more than delivering results, allowing a politician to remain forever “promising” without ever having to become “proven.” It’s a compact satire of electoral incentives: campaigns and public approval often hinge on optimistic commitments, while accountability for fulfillment can be delayed, diffused, or avoided. The line also hints at cynicism about political careers as performances sustained by rhetoric rather than accomplishment.