Ta'anis 7a: Anyone who engages in Torah Lishmah, it becomes for him an elixir of life.

Rashi: It is life for those who hold it. This is like "va'Yachazek Bo" (Shemos 4:4).

Malbim: A Kovesh's nature leans towards evil, just he is Kovesh (conquers) his Yetzer. This is Machazikim Bah. He holds [Chachmah] Bal Korchah. It is as if Chachmah wants to depart from him, for his nature opposes it, but he does not let it leave, for he overpowers his Yetzer. The verse need not teach that he has happiness of Nefesh. His happiness and reward will be according to his great pain to conquer his Yetzer. Reward is according to the pain! One might have thought that his inner war to conquer his Yetzer shortens his life. Therefore, it says "Etz Chayim Hi" - it is as if he ate from Etz ha'Chayim, and will live forever.

Mishnas R. Aharon (4, p.34, 37): This refers to those who hold onto Torah to save themselves from descending to the pit, like one who fell in a raging river holds onto a tree. This is a level of Lishmah.

Rashi: They are those who come close to it. Every Temichah in this Sefer is holding the matter.

Malbim: The nature of a Tzadik's heart leans towards Chachmah. His Yetzer does not entice him to transgress its laws. He need not hold Chachmah Bal Korchah; it suffices to support it. One might have thought that he does not have so much reward, for he had no inner war - therefore, it says "Me'ushar".

Mishnas R. Aharon (4, p.34, 37): A higher level is those who want that also others should learn. They engage in Sugyos that few learn, amidst love, for they know that Torah is pained when people do not engage in it. They want to support Torah itself. They are happy, for the Torah bestows double to them.