Why does light of Tzadikim make rejoice?
Chagigah 12a: This is the light that Hashem created on the first day. With it, one can see from one end of the world to the other end. Once Hashem [fore]saw Resha'im, He hid it from them, and it will be for Tzadikim in the world to come; they will rejoice in it.
R. Yonah: A Tzadik's Nefesh resembles light. It lasts forever, like light, and its existence does not depend on anything other than Hashem, just like sunlight depends only on Him. A Tzadik's Nefesh always rejoices, for also in this world it rejoices in Mitzvos. It afflictions come on it, it rejoices in afflictions. In the world to come it will have Simchah without end. So the sun always rejoices to do Hashem's will - "Yasis k'Gibor Larutz Orach" (Tehilim 19:6). It says about one who serves Hashem from love and is happy to receive afflictions "v'Ohavav k'Tzeis ha'Shemesh bi'Gevuraso" (Shoftim 5:31). (Gitin 36b).
Malbim: "Ohr" is light itself, e.g. of the sun, which constantly illuminates. It does not need oil or a wick. Tzadikim's Neshamos constantly illuminate; "Ohr Zaru'a la'Tzadik" (Tehilim 97:11). It does not need a lamp (body). The spiritual Nefesh always illuminates with spiritual light. Spiritual matters constantly rejoice, for they are eternal - "ul'Yishrei Lev Simchah", "Simchu ba'Shem" (ibid., 32:11). Anything that perishes is sad, for it knows that its end is destruction.
What is "[v'Ner Resha'im] Yid'ach"?
Rashi: It is an expression of jumping - the flame jumps and is cut off.
R. Yonah: It will extinguish. [Resha'im's] light is like a lamp; its light depends on oil or wax. When they are finished, it extinguishes. So the Rasha's Nefesh depends on the body's existence. It is happy with the body and sometimes succeeds in this world, and when the body is finished, also its Simchah ceases, for it goes to destruction.
Malbim: A Ner needs oil and a wick. The light of Resha'im's physical success is only in this world (in the life of the body). Once the oil and wick are finished, the lamp extinguishes and the success is lost. This refers also to "Kofer Nefesh Ish Ashro" (8) - he must give life to the Nefesh, which is perpetual life, and not to the body and the light of its success, which will extinguish. One who gets rich in this [latter] way, he has nothing (refer to 13:7:1:4).
