Doing things in a joyful manner has a tremendous effect. This is evident from the verse which says that the punishments of exile came, "because you did not serve Hashem with joy." Additional joy therefore nullifies the exile.
Eve Of The 12th Day Of Sivan, 5743
One thing only: One must be firm in one's trust, and one must do so joyfully. For it is taught in the Zohar, Parshas Tetzaveh (page 184b): This lowly world of ours receives its entire flow of beneficence from Above. If down here [people live their lives] with joy and with light, then the world Above reciprocates in kind, with light and with joy. Moreover, it is written that one must "serve G-d with joy." Hence, if a Jew is joyful, this joyfulness down here calls down upon him a corresponding joy from Above.
Igros Kodesh, Vol. 5, p. 204, Letter 1405
Despite the darkness of Galus we can “serve G-d with joy,” and serve Him in a manner which encompasses every moment of our lives and every aspect of our behavior, even our sleep.
On the surface, it seems impossible for us to be happy when we are living in Galus. In the midst of our service to G-d we are conscious of Galus. However, we must realize that our service consists of turning darkness into light, and bitterness into sweetness, and that through our service we will bring about the revelation of the messianic age. Furthermore, our service, which brings about the Messianic revelation, possesses an advantage over the revelations itself, as our sages declared, “one moment of Teshuvah and good deeds in this world is better than the entire life of the world to come.” We must view ourselves as a person who owns a locked chest of precious jewels and also possesses the key. From this we can feel great joy.
Eve Of Rosh Chodesh Nissan, 5740
The Jews should rejoice in their Maker. Every Jew should share in G-d's joy, who rejoices and is happy in His dwelling in this world. (Tanya ch. 33)
This means that G-d is happy with the world He made and Jews should be doubly happy for this. On this day the Rashbi also asked us to be more happy. When a person is happy he can accomplish a lot more than usual, so that from this day we go forward with rich optimism, certain of growth and accomplishment in Torah and mitzvos, with great happiness, and then G-d will bestow all His blessings upon us.
When Jews serve G-d with joy it increases G-d's joy. When a Jew rejoices that G-d is with him then G-d in even happier with His people. And through His people G-d is also happy with all His creations – the heavens, the earth and all their host which are blessed and continue to exist in the merit of "the breath of small children in which there is no sin." The Universe will then be blessed with the supreme blessing of peace which includes all other blessings – and engenders all other blessings, brilliantly and openly.
Shabbos Parshas Emor, 17th Day of Iyar, 5747
Being truly alive means that one's life should lack nothing of whatever you and your wife need materially and spiritually. However, it can happen (G-d forbid) that perhaps one does not deserve to receive such blessings from the Holy One, blessed be He. Concerning such a situation it is written in the holy Zohar: "If, from down here below, a person shows a luminous countenance, in the same way does a luminous Countenance shine upon him from Above [...]. In this spirit it is written, 'Serve G-d with joy': the joy of a mortal elicits upon himself another, Supernal joy. Similarly the world below, being thus crowned, draws down [blessings upon itself] from Above."
Igros Kodesh, Vol. 6, p. 266, Letter 1773