He does not give what His saints seek on the surface of their hearts and with that foam of words, but He is an almighty and exceedingly rich Bestower who gives in accordance with the depth of that sighing. Therefore He lets prayer be directed, grow, and be increased; and He does not hear immediately. For if He were to answer at the first outcry or petition, prayer would not increase but would become cold. Therefore He defers help. As a result, prayer grows from day to day and becomes more efficacious. The sobbing of the heart also becomes deeper and more ardent until it comes to the point of despair, as it were. Then prayer becomes most ardent and passionate, when it seems that now the sobbing is nearly at a standstill.
But if He heard immediately, prayer would not be so strong, so alive, and so ardent; but it would be only a superficial and pedagogical sobbing which is still learning to pray, to sob, and to desire, and is not yet a master of prayer. But when the point of despair has been reached and the afflicted heart thinks: “Alas, nothing will come of it; all is lost!” yet a spark and a dimly burning wick remain (cf. Is. 42:3), then be strong and hold out. For this is the struggle of the saints who think that the rope will now be torn yet who continue to sob. Then, therefore, prayer is perfect and strongest.
– Luther on Genesis 30:24 (AE 5)