For example, the Talmud (Shabbat 30b) relates how Rava opened a lecture by telling jokes, leading his rabbinic audience to laugh. Afterwards he turned serious and started the heart of the lesson. Rava started by telling jokes, so that the rabbis would be joyous, as the Almighty's spirit rests only out of happiness.
This lesson applies likewise to today's reality, setting the course on how rabbis and teachers should address their pupils. In order to bridge gaps, the teacher must touch his pupils' souls - which cannot tolerate sadness. By no means should the teacher come across with anger.