From Psalm 13 in "The Treasury of David" by Spurgeon

Though this is a little long for a quote, it's worth the read.

'Verses 1, 5, 6.--Prayer helps towards the increase and growth of grace, by drawing the habits of grace into exercise. Now, as exercise brings benefit to the body, so does prayer to the soul. Exercise doth help to digest or breathe forth those humours that clog the spirits. One that stirs little we see grow pursy [fat/pudgy], and is soon choked up with phlegm, which exercise clears the body of. Prayer is the saint's exercise-field, where his graces are breathed; it is as the wind to the air, it brightens the soul; as bellows to the fire, which clears the coal of those ashes that smother them. The Christian, while in this world, lives in an unwholesome climate; one while, the delights of it deaden and and dull his love of Christ; another while, the trouble he meets in it damps his faith on the promise. How now should the Christian get out of these distempers, had he not a throne of grace to resort to, where if once his soul be in a melting frame, he (like one laid in a kindly sweat), soon breathes out the malignity of his disease, and comes into his right temper again? How often do we find the holy prophet, when he first kneels down to pray, full of fears and doubts, who, before he and the duty part, grows into a sweet familiarity with God, and repose in his own spirit! (Psalm xiii. 1) He begins his prayer as if he thought God would never give him a kind look more: "How long wilt thou forget me, O Lord? for ever?" But by that time he had exercised himself a little in duty, his distemper wears off, the mists scatter, and his faith breaks out as the sun in its strength, verses 5, 6: "I have trusted in thy mercy; my heart shall rejoice in thy salvation. I will sing unto the Lord." Thus his faith lays the cloth, expecting a feast ere long to be set on: he that now questioned whether he should ever hear good news from heaven, is so strong in faith as to make himself merry with the hopes of that mercy which he is assured will come at last.' - William Gurnall