Eccl. 2:1. I said to myself: Come now, I will make a test of pleasure; enjoy yourself. But behold, this also was vanity.

In the preceding chapter he showed that the anxiety and solicitude and planning of men, particularly when we want to be of aid and counsel to others, are vain and empty. He teaches that we should be content with the Word and work of God, so that we are not presumptuous about anything except what the Word of God prescribes or His work requires. Often this seems to be contrary to the Word, as when He oppresses us in some way even though we have acted correctly according to the Word. Then we need faith and patience, so that we do not seek our own counsels and try to evade what He has sent, but commit ourselves to Him and bear the hand of God in His work. Now he turns to the other side. (Luther’s Works, v. 15, p.29)

And here we can put these four things, as also above, namely:
Rejoicing at the words of God in the spirit,
Being sad according to the cross in the flesh,
Rejoicing at the reality of the world in the flesh,
Being sad at the cross in the conscience or spirit.
Joy inwardly over the Word.
Joy outwardly over the reality.
And
Sadness externally over the lack of the reality.
Sadness inwardly over the lack of the Word.
Therefore happy is the sadness of the faithful, which has such joy with it, and unhappy is the joy of the unbelievers, which has such sadness with it. This joy of the former is founded on the Word of the Lord, which abides forever, while the joy of the latter is founded on the flesh, which is grass because its flower has dried up and fallen (Is. 40:7). (Luther’s Works, v. 11, p.521)

This also must be noted, that as the Word of God appears to the carnal to be lacking much and in many things, so it appears to the spiritual to abound in all things like many spoils. As in the figure of the children of Israel, who seemed to be lacking everything in the wilderness, so that even the manna was sickening to them, complaining that they were in a desert of barrenness, so it is necessary for the hearers of the Word of God to be led into the wilderness where there is nothing that the flesh demands. But when the flesh has been crucified and all that is of the flesh has been removed, only the spirit should be fed and led into the land of promise, that is, the spiritual man.

Therefore the carnal man necessarily murmurs here, because he finds nothing that pleases him. Thus the more carnal and worldly a man is, the more he has to bring up against the Word of God. And what wonder? Since it is altogether foolishness to him, he knows nothing useful in it. Therefore great is the praise of the church, which has the Spirit so abundantly that it esteems only the words of God, to rejoice in them above all spoils. Rejoicing in those things which to others seem to be sad things and nothings without a doubt puts a great void between the latter and the former. (Luther’s Works, v. 11, p.520-521)