From The Rare Jewel of Christian Contentment by Jeremiah Burroughs

But now, my brethren, because this discontented humor is tough, and very hard to word upon-there is none who is discontented but has something to say for their discontent—I shall therefore seek to take away what every discontented heart has to say for himself.

1. ONE THAT IS DISCONTENTED SAYS, 'IT IS NOT DISCONTENT; IT IS A SENSE OF MY CONDITION.' I hope you would have me sensible of my condition. Perhaps when God takes away a friend or some other comfort, they are inordinately sorrowful, and wringing their hands as if they were undone; but let anyone speak to them, and they say, 'Would you not have me sensible of my affliction?' Thus many would hide their sinful murmuring under God's hand with this pretense, that it is but sensibleness of their affliction. To that I answer:
1. There is no sense of any affliction that will hinder the sense of God's mercies. Nay, the more we are sensible of our afflictions, providing it is in a gracious manner, the more sensible we will be of God's mercy. But you are so sensible of your affliction that it takes away the sense of all your mercies. Oh, this is sinful discontent, this is not to be sensible in a wicked way, you go beyond your bounds. By this rule you may come to know when your sorrows and troubles for your afflictions go beyond the bounds.

We may be sorrowful when God afflicts, but, oh, that I might know when my sorrow goes beyond the bounds of it! Truly, you may know it by this, does the sense of your afflictions take away the sense of your mercies? If it does, then it goes beyond the bounds.

2. If it were but a bare sense of an affliction it would not hinder you in the duties of your condition. The right sense of our afflictions will never hinder us in the performance of the duties of our condition; but you are so sensible of the affliction that you are made unfit for the performance of the duties of the condition that God has put you in. Surely it is more than mere sense of your affliction!

3. If it were but a mere sense of your affliction, then you could in this your condition bless God for the mercies that others have; but your discontentedness usually breeds envy at others. When anyone is discontented with their condition, they have an envious spirit at the conditions of those who are delivered from what afflictions they bear.

Certainly, then, it has turned sour when you are so sensible of your afflictions and insensible of mercies that you are unfit for the duties of your condition, and envious of others who are not afflicted as you are.