I have said this to explain the stanza that follows, in which the soul replies to those who call in question its holy tranquillity, who will have it … - John of the Cross

In this night the soul obtains these four benefits here mentioned, namely, delight of peace, constant remembrance of God, purity and cleanness of soul, the practice of all the virtues of which I have just spoken. So David speaks from his own experience when he was in this night. “My soul,” he says, “refuses to be comforted; I was mindful of God and was delighted, and was exercised, and my spirit fainted.” He adds forthwith: “I meditated in the night with my own heart, and I was exercised, and I swept my spirit”6 clean of all affections.

But it may be asked: Why does the soul call the divine light, which enlightens the soul and purges it of its ignorances, the dark night? I reply, that the divine wisdom is, for two reasons, not night and darkness only, but pain and torment also to the soul. The first is, the divine wisdom is so high that it transcends the capacity of the soul, and therefore is, in that respect, darkness. The second reason is based on the meanness and impurity of the soul, and in that respect the divine wisdom is painful to it, afflictive and dark also.

Happiness is not a destination, it's a method of travel.