The following encouragement came in a letter from Robert Cushman to the William Bradford recorded in Of Plymouth Plantation: 1620-1647 (page 108):
I pray you not be discouraged, but gather up yourself, to go through these difficulties cheerfully and with courage in that place wherein God hath set you, until the day of refreshing come.
Regarding John 15:16,
that the greater part of teachers either languish through indolence, or utterly give way through despair, arises from nothing else than that they are sluggish in the duty of prayer.
—John Calvin, Commentary on John, 122
John Calvin remarks on the rage of the nations and the response of Yahweh in Psalm 2:4.
[W]hen God permits the reign of his Son to be troubled, he does not cease from interfering because he is employed elsewhere, or unable to afford assistance, or because he is neglectful of the honour of his Son; but he purposely delays the inflictions of his wrath to the proper time, namely, until he has exposed their infatuated rage to general derision.
—Commentary on the Book of Psalms
In other words, God has presidents and prime ministers right where He wants them to show off their ridiculous foolishness.
In light of all the reasons God gives us to be grateful–as a society, a church, in our families, and on the nitty-gritty of our dinner tables–why do so many grumble? We whine because we want to be God. Here’s a dependable observation from a dubious source, Friedrich Nietzsche.
There is a powerful causal drive within [man]: someone must be to blame for feeling bad…And waxing indignant makes him feel better, too: all poor devils take pleasure in cursing, it gives them a little rush of power. (Twilight of the Idols)
In other words, complaining is the closest we get to being God. We know that we can’t actually control anything but if we complain about it, then we can at least make ourselves feel like we are above the problems. Only those who aren’t trying to be God can thank God for whatever He gives. Otherwise we’ll just be fussy devils.