On this daf, we find that an avel is forbidden to learn Torah, since Torah gladdens the heart.
The Iglei Tal writes in his introduction to hilchos Shabbos that the main mitzvah of talmud Torah is “to take pleasure and joy in one’s learning. Learning in this way causes the words of Torah to be absorbed in one’s blood.”
In a similar vein, Rav Shlomo Wolbe writes in a letter, “The main way to develop ahavas Hashem is that one feels such great pleasure in every word of Torah and emunah that this Torah learning is literally the ultimate pleasure for a person – tachlis hahana’ah.” (He understands this from the Rambam in Sefer Hamitzvos.)
Anything learned in this spirit draws down tremendous holiness. Of course, some people take pleasure specifically in a particular sefer or method of learning. The Mishnas Chassidim writes that one who feels drawn to learn a particular type of learning should spend most of his time doing this, since usually feeling a draw to some particular holiness signifies that this is likely his personal rectification.
The Mishnas Chassidim adds that one should learn some of each major category of Torah, yet one should still spend most of his time learning what speaks to him.
Yeshivos focus mostly on learning in depth, since without proper analytical skills, one will lack true comprehension of what he learns. Another reason for this is because this sort of learning is easier to get into and enjoy, since it is intellectually stimulating (Maaseh Ish; Michovim Ukesavim; Shnas Chassidim).