Musar su Salmi 19:9

And there are some who rationalize thus: "What good will it do him to be a [Torah] learner? Is he gong to be a Rabbi?" (the implication being that if he is not going to be a Rabbi, a Jew need not know how to serve the L-rd, G-d forbid!) Let there not be such in Israel. Are we not familiar with the verse (Psalms 19:9): "The mitzvah of the L-rd is clear, enlightening the eyes," which is understood to mean that a man, without Torah to guide him, is like a blind man, walking by himself on a road full of pits, whose danger is extremely great, and whose failure is certainly nearer than his escape. So, exactly, is a man devoid of Torah. Not only will he be lacking many hundreds of mitzvoth because of this [ignorance of Torah], not knowing at all if what he is doing is a mitzvah or not, but he will commit in their stead many hundreds of transgressions which he thought to be mitzvoth. And, even more, the mitzvoth that will remain with him, aside from most of them being performed without joy, without love, and without fear, will themselves be imperfect and defective, for he will not know how to perform them according to the din.

There is another good in joy as in the case of two men of whom it was said that they would have a share in the world to come because they were happy men and whenever they saw a sad man they would cheer him up, and whenever they saw two men quarreling with each other they would tell them humorous stories until they brought peace between the quarreling men. And this is true in discussions of the law, the Sages would begin with humorous words to open the mind to study with joy. Of course, this does not mean vulgar humor or nonsensical jesting, but rather words concerning the precepts of the Torah which cause the heart to rejoice as it is written: "The precepts of the Lord are right, rejoicing the heart" (Ps. 19:9). And all this pertains to the joy of fulfilling the commandments.