Chasidut על שבת 60:6

For example, the Gemara (Shabbat 30b) says, "Rabbah, before he began teaching to the sages, would say something humorous and the sages would be cheered. Ultimately, he sat in trepidation and began teaching the topic." To here are the words of the Gemara. He said something humorous so they would be happy, since the Divine Presence only resides amidst joy - as the Gemara says there. However the Torah is like a hammer that breaks apart a boulder which lets off many sparks and many explanations. So in our generation, the Gemara is also hinting to a path for the rabbi or the teacher: The teacher must make efforts to win over the heart of the student and to keep bringing him close. And joy is one of the main vehicles through which he can win him over. The spirit of a child does not tolerate sadness, and the teacher should never appear to him like an irritable or disputative person.