A thankful spirit is a source of joy, whereas an unthankful spirit is a source of sadness. Being grateful for a gift preserves the gift, otherwise we will lose it. Also, when tested by some trial you should try to find out not why or through whom it came, but only how to endure it gratefully, without distress or rancor.
Based on St. Mark the Ascetic
In the photo, a happy and thankful monk displays fruit of the olive harvest.