"…for the reward of a mitzvah is a mitzvah…." (Avot 4:2)

The reward for a mitzvah is brought about by the mitzvah itself. (Tanya ch. 37)

The reward given to a person for performing a mitzvah is not the same as payment given to a worker for doing a job. A worker plows and sows, etc., and the owner of the field pays him money for his labor. However, the worker does not create the money he is given as his wage. However, in our case, the mitzvah itself creates its reward. (And the same is true of a transgression). [The Lubavitcher Rebbe in Likutei Pirushim l'Tanya, p. 740 (3)]

When one performs a mitzvah its radiance is drawn down into Gan Eden….

The reward and delight a soul receives in Gan Eden is radiated by the mitzvah itself. And this is the meaning of the verse, "He placed him [Adam] in Gan Eden to work [literally, 'to make it'] and safeguard it" - i.e. Torah and mitzvot make Gan Eden by drawing down the radiance of the Ohr Ein Sof into Gan Eden, and in this he delights. This is also the reason that the mitzvot are being called "candles" - when one lights candles, the house becomes filled with light; when one performs a mitzvah its radiance is drawn down into Gan Eden. [Rabbi Shalom Dovber Shneersohn, Ma'amarim 5672, p. 1219 (3)]

One who fulfills mitzvot out of habit…will earn only a superficial level of delight….

Earning Gan Eden through Torah and mitzvot is not like payment for a job. A worker who did not do his job properly will not be paid by his employer who is unwilling to pay for a job which has not been done. However, one who does not do a mitzvah will automatically remain without reward, for the reward of the mitzvah derives from the mitzvah itself. [Tzemach Tzedek's Ohr HaTorah, Chukat, p. 863(3)]

All tzadikim sit in Gan Eden with a crown on their heads and delight in the radiance of the Shechinah - this is the delight experienced in comprehending revelation of G‑dliness. However, since the reward of a mitzvah is the mitzvah itself, it is proportional to the effort a person expended in doing the mitzvah. Hence, each person is worthy of divine revelation only according to the efforts he made in Torah and mitzvot in this world. One who accepts the yoke of Heaven upon him in this world will delight in the revelation of G‑dliness. But one who fulfills mitzvot out of habit, without awareness and concentration, will earn only a superficial level of delight. [Likutei Torah, Devarim, 78a; Ma'amarei Admor HaZaken on Torah, p. 898 (3)]