When one puts his fate in God’s hands and relies, from the depths of his soul, only on Him, God treats him with generosity even if he is unworthy. This is the basis for the hasidic phrase, “Think good and it will be good.”

One must cast his fate into God’s hands:

The commandment to trust in God…is itself a service of God. It means that a person must rely on the Holy One, blessed be He, to the point that he casts his entire lot into God’s hands, as it is written, “Cast your burden upon the Lord” (Psalms 55:23); and that he relies on nothing other than God.

When one relies on God, God is generous toward him:

This itself is the foundation of a person’s trust in God – that the Holy One, blessed be He, will do good for him, palpable and revealed good, even if he is unworthy of this kindness…. If a person truly, from the depths of his soul, relies only on the Holy One, blessed be He, such that he is not concerned about anything, then this awakening itself causes the Holy One, blessed be He, to behave toward him in this manner, doing good for him (even if without this trust he is unworthy of it).

Think good and it will be good:

This is the meaning of the statement of the Tzemaĥ Tzedek, “Think good and it will be good.” Trust in God itself brings about a positive outcome. This is not an incidental element of trust, but is the very definition of trust, concerning which we were commanded. (Rabbi Menaĥem Mendel Schneerson, Likutei Siĥot 36, p. 1)

The Ba’al Shem Tov was once traveling with the saintly Rabbi Mendel of Be’er. During the journey, Rabbi Mendel became thirsty. The Ba’al Shem Tov said to him, “If you have complete trust in the Holy One, blessed be He, water will certainly appear.”