St. Isaac the Syrian: On thankfulness to God

The thanksgiving of the recipient incites the giver to give gifts greater than the first. He that returns no thanks in small matters is a dissembler and dishonest in greater ones also. If a man is ill and recognizes his ailment, his healing will be easy. If he confesses his pain, he draws nigh its cure. The pangs of the unyielding heart will be multiplied, and the patient who resists his physician amplified his torment. There is no unpardonable sin, save the unrepented one. Nor does any gift remain without addition, save that which is received without thanksgiving. The fool's portion is small in his eyes.
+St. Isaac the Syrian, from The Ascetical Homilies, Homily 2 On thankfulness to God, in which there are also essential elementary lessons.