“Cognition, knowledge, no longer appears to me as an end in itself. It has turned into service . . . Cognition is autonomous, it refuses to have any answers foisted on it from the outside. Not every question seems to me to be worth asking . . . now I only inquire when I find myself inquired of . . . I am anxious to answer the scholar qua person but not the representative of a certain discipline, that insatiable, ever inquisitive phantom which like a vampire drains one whom it possesses of their humanity. I hate that phantom as I do all phantoms. On the other hand, the questions asked by human beings have become increasingly important to me”
Franz Rosenzweig, God, Man, and the World. Lectures and Essays, 26
Knowledge, like few other resources, is not diminished as it is given away. Overall knowledge is increased as it is "consumed".Sharing knowledge reinforces your command of it, that is as you pass knowledge you gain more knowledge. To fully appreciate that, try teaching some one something you just learned vs something you've always known.