opportunity only knocks once

proverb Great opportunities are typically only encountered once. The phrase implies that one risks losing them forever if one does not act quickly and seize them right away. Opportunity only knocks once—if you don't accept the job in Japan now, you'll never go! A: "I'm nervous about committing to grad school. I know it would open all sorts of doors for me, but I'd have to move away from all my friends and family." B: "Opportunity only knocks once, dude. You'll always regret it if you don't go for it."