It’s not that you should be selfish sometimes, but that you should always be selfish.
Selfishness is key to success and happiness. Those who put their own needs first are more likely to achieve their goals. And once they’ve achieved them, they’re in a better position to enjoy life and help others.
Being selfish doesn’t mean being cruel or heartless. It means doing what’s best for yourself, even if it means disappointing others at times. So go ahead and be selfish—it won’t kill you (and it just might make you happier).