Teaching Torah is the greatest chesed (kindness) one can bestow upon another person. If you give someone food or money you are helping him for a limited amount of time. But if you teach someone Torah you are helping his soul for all eternity.
To appreciate this, we must understand the concept of eternity. If the entire earth were covered with sand one mile high and every million years one grain would be removed, it would take an enormous amount of time for all the sand to be removed. But this is not eternity, for eventually none of it would remain. Eternity is without end (Rabbi Y. Kaniewsky in Chayai Olam, vol 2, ch.5).
Therefore, a person should cherish the opportunity to teach Torah to others.