See Taz Siman 524 sk 1(1) (I will not say exactly the same thing but reading the Taz I think an answer to the question). He describes a situation in which the pupils don't have pleasure with the study but the teacher has, and prohibit because the pleasure of the teacher. the teacher is adult and the pupils are children, the teacher is accustomed with the Gemara and Children not. Here is a Group in which a minority will enjoy and the majority will not. The learning is prohibited for the minority. But this minority is the generator of the learning and cannot be skipped, if it was possible, the learning should be organized without this minority. So if a person enjoy to learn the sad topics, he cannot learn. This proof is not really a proof because the group cannot be uniform when for the teacher and the pupils the topic represent something else. Anyway Svara Peshuta that to enjoy at hight degree is the problem, not the learning. So any enjoyment should be avoided.
(1) נראה פשוט דפירוש איוב ושאר דברים המותרים לגדול מותר ללמוד גם עם התינוקות וכאן לא אסרו אלא הלימוד שהקטנים לומרידם בסדר שלהם דהיינו חומש וגמרות וקמ"ל דאע"ג דיש צער קצת בלימוד עמהם ור"מ ס"ל בגמרא דהותר לשנות במה שאינו רגיל ואע"ג דלא קי"ל כוותיה היינו בלימוד של עצמו אבל עם התינוקות ה"א דשרי קמ"ל דאפ"ה ליבטלו מזה ואיסור של התינוקות בלימוד אין הטעם משום שהם שמחים דודאי אין להם שמחה אלא משום המלמד ששמח בד"ת שלומד עמהם כנ"ל.
Translation of the content: I seems obvious to permit to learn with children comments on Yiov and last things allowed for adult. Why is it prohibited to learn with children? Children doesn't become happy from the study! The reason is the teacher. He would be happy when learning Torah with them. You can ask: We already know that The Halacha is ruled against Rabbi Meir (Who stated that a study which is hard because new is permitted) because all study will lead the student to happiness when he finally understand!? The answer is that when there is a stronger reason to not be happy, The hardness to teach children and the teacher will be allowed to learn Chumash and Gemara whith children. The conclusion of the halacha is that even in these conditions, the teacher is at least a bit happy to learn Torah words with them.
Some people asked a question concerning the fact that to learn Job and the sad subject may be interesting. I remember a Mishna in Yoma. The Mishna expose how they made in time of Bet Mikdash to avoid that Kohen Gadol fall asleep.
- If he was a sage, he would expound.
- and if not, the disciples of the sages would expound before him.
- If he was familiar with reading, he would read.
- If he was not a sage and nor familiar with reading, they would read
before him, from Job, Ezra and Chronicle.
וּבַמֶּה קוֹרִין לְפָנָיו, בְּאִיּוֹב וּבְעֶזְרָא וּבְדִבְרֵי הַיָּמִים.
Rashi explains that they stimulate interest and the Kohen Gadol will stay awake.
So, to read Job is known as interesting. But if you see the list in the Mishna, there is a descending order of interest.
In conclusion, the degree of interest which is prohibited is not a simple interest of reading, but an interest of Chidushe Suggiot.