From Letter to Marcellinus, by Athanasius:
Son, all the books of Scripture, both Old Testament and New, are inspired by God and useful for instruction, as the apostle says; but to those who really study it the Psalter yields especial treasure. Within it are represented and portrayed in all their great variety the movements of the human soul. It is like a picture, in which you see yourself portrayed and, seeing, may understand and consequently form yourself upon the pattern given. In the Psalter you learn about yourself. You find depicted in it all the movements of your soul, all its changes, its ups and downs, its failures and recoveries. Moreover, whatever your particular need or trouble, from this same book you can select a form of words to fit it, so that you do not merely hear and then pass on, but learn the way to remedy your ill. Prohibitions of evildoing are plentiful in Scripture, but only the Psalter tells you how to obey these orders and refrain from sin.
But the marvel with the Psalter is that, barring those prophecies about the Savior and some about the Gentiles, the reader takes all its words upon his lips as though they were his own, written for his special benefit, and takes them and recites them, not as though someone else were speaking or another person’s feelings being described, but as himself speaking of himself, offering the words to God as his own heart’s utterance, just as though he himself had made them up.