Gemara: [Regarding the apparent contradiction over whether it is proper for there to be lightness or only seriousness surrounding Torah learning,] there is no question – this is for the teacher (proper), this is for the student (improper). Another possibility is that both refer to the teacher – one is before beginning to learn (proper), one is after beginning. This was practiced by Rabba who would say something humorous before starting and then would sit with trepidation and begin the lesson.

Ein Ayah: [We have already seen that the enjoyment of Torah study is to be experienced only after one has made it through the early stages, which require much hard work.] It is thus understood that the teacher, who has already reached the point of being able to readily appreciate the experience, can be involved in its enjoyment. Although this includes opening himself to thoughts that come from and can pull one in various directions, he is already at the point of clarity of mind that he can handle it. To the contrary, the broadness is something that will enable him to give over the Torah in a more impressive and impactful manner.
According to the second version of the gemara, even the teacher must deal with happiness and humor only before he starts teaching. Even though this can be helpful and he should be stable in his mastery, if he is truly great, he will always be striving for an even higher and deeper level. Even on topics in which he has reached the mastery to teach others, he still wants to reach new levels of understanding, and this requires that he maintain the seriousness that allows him to focus in a confined manner. Despite his already acquired love for Torah knowledge in and of its own right, he still wants to act like a student at the time of his learning.
The reason to choose happiness or seriousness depends on the way one views the knowledge that he has already acquired. On the one hand, by the time he has come to teach, the matters should have become part of his nature, and this brings joy. On the other hand, the feeling that acquiring wisdom is an inexhaustible process causes him to feel seriousness. This seriousness does not erase the spark of joy that stems from satisfaction at what has been attained. To the contrary, the happiness when one begins to teach, celebrating that which he has reached, gives him incentive to continue to attain more and more.
Not only does the teacher have what to enjoy, but it is also educational to share this with his students, so they can see that which they are able to attain themselves. That glimpse helps the students deal with the process of great toil they have to pass through. Therefore, the teacher can include them in the humorous matter. After using that to prepare themselves for the yoke of disciplined learning with the right spirit, the teacher sits in trepidation, as one who knows what is missing. It is even clearer that the students need to sit in trepidation, to go about the strenuous work they need until they reach the treasure of knowledge. This is the trepidation of anticipation of one who goes towards a very beloved and awaited goal.