Ecclesiastes 1:9

Two things we are apt to take a great deal of pleasure and satisfaction in, and value ourselves upon, with reference to our business and enjoyments in the world, as if they helped to save them from vanity. Solomon shows us our mistake in both. 1. The novelty of the invention, that it is such as was never known before. How grateful is it to think that none ever made such advances in knowledge, and such discoveries by it, as we, that none ever made such improvements of an estate or trade, and had the art of enjoying the gains of it, as we have. Their contrivances and compositions are all despised and run down, and we boast of new fashions, new hypotheses, new methods, new expressions, which jostle out the old, and put them down. But this is all a mistake: The thing that is, and shall be, is the same with that which has been, and that which shall be done will be but the same with that which is done, for there is no new thing under the sun, Ecc 1:9. It is repeated (Ecc 1:10) by way of question, is there any thing of which it may be said, with wonder, See, this is new; there never was the like? It is an appeal to observing men, and a challenge to those that cry up modern learning above that of the ancients.

— Matthew Henry's Concise Commentary (public domain)

Rather, "no new thing at all"; as in Num 11:6. This is not meant in a general sense; but there is no new source of happiness (the subject in question) which can be devised; the same round of petty pleasures, cares, business, study, wars, &c., being repeated over and over again [HOLDEN].

— Jamieson-Fausset-Brown Commentary (public domain)