Zohar Tetzaveh P184b.
.... in addition to all the statements of our Sages explaining how [being depressed] is undesirable, it also, chas v’shalom, ruins the channels – [The channels of blessing flowing from above].
To refer to the wording of the Zohar, Parshas Tetzaveh, p184b: “Ta Chazi – Come, see: ....The upper world does not give [shefa] to the lower world but only according to its demeanor [attitude]. If the world below is poised with a happy face [attitude], happiness is beamed to it from above. And if it [the lower world] projects a sad demeanor, [the upper world] sends din [judgment, etc.] its way. This is what it says (Tehillim Ch.100), “Serve Hashem with joy.” A person in a state of happiness draws to himself more happiness from above.” ...
In another letter, the Rebbe quotes the above Zohar and adds that “It is important that anash know this.”
==== Part of a letter: Igros Kodes V4 p34. Chabad.org/happiness as a conduit for blessing
