Charles H. Spurgeon on Psalms 119:142

Thy righteousness is an everlasting righteousness. Here the law of God is honoured by the additional encomium, that it is everlasting righteousness and truth; as if it had been said, that all other rules of life, with whatever attractions they may appear to be recommended, are but a shadow, which quickly vanishes away. The Psalmist, no doubt, indirectly contrasts the doctrine of the law with all the human precepts which were ever delivered, that he may bring all the faithful in subjection to it, since it is the school of perfect wisdom. There may be more of plausibility in the refined and subtle disquisitions of men; but there is in them nothing firm or solid at bottom, as there is in God's law. This firmness of the divine law he proves in the following verse from one instance the continual comfort he found in it when grievously harassed with temptations. And the true test of the profit we have reaped from it is, when we oppose to all the distresses of whatever kind which may straiten us, the consolation derived from the word of God, that thereby all sadness may be effaced from our minds. David here expresses something more than he did in the preceding verse; for there he only said that he reverently served God, although from his rough and hard treatment he might seem to lose his labour; but now when distressed and tormented, he affirms that he finds in the law of God the most soothing delight, which mitigates all griefs, and not only tempers their bitterness, but also seasons them with a certain sweetness. Assuredly when this taste does not exist to afford us delight, nothing is more natural than for us to be swallowed up of sorrow. John Calvin.