Sharing what you know can be selfish if the information intends to impress others rather than help. Being a know-it-all is an independent service.
But if you talk with teaching in mind, out comes a lesson. It’s selfish to keep that information all to yourself.
Everyone’s a teacher when they express what they know and impart their knowledge with the world.
Teachers also follow up with care to answer any questions.
A good service is willing to repeat itself.