A sign of true repentance is the acknowledgment that we deserve all the afflictions, visible and invisible, that come upon us, and even greater ones.

St. John Climacus

Grumbling is caused by misery and it can be put aside by doxology (giving praise). Grumbling begets grumbling and doxology begets doxology. when someone doesn’t grumble over a problem troubling him, but rather praises God, then the devil gets frustrated and goes off to someone else who grumbles, in order to cause everything to go even worse for him. You see, the more one grumbles, the more one falls into ruin. 

Sometimes the devil deceives us and makes us unable to be pleased with anything; however, one can celebrate all things in a spiritual manner, with doxology, and secure God’s constant blessing.

St. Paisios of Mount Athos