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"Limitations" tastes sour in the mouth. For starters, we don't like limitations for ourselves: some of our most exciting stories are people transcending the "limits" imposed by the known world. Breaking free of gravity to enter space. Conquering distance by the telegraph, telephone, and now satellites. Scout Finch pushing the limits of what society accepts.
And those are exciting - there are limitations we can or should push - but not all boundaries were made to be tested. As G.K. Chesterton wrote,
The moment you step into the world of facts, you step into a world of limits. You can free things from alien or accidental laws, but not from the laws of their own nature. You may, if you like, free a tiger from his bars; but do not free him from his stripes. Do not free a camel of the burden of his hump: you may be freeing him from being a camel. Do not go about as a demagogue, encouraging triangles to break out of the prison of their three sides. If a triangle breaks out of its three sides, its life comes to a lamentable end. (from Orthodoxy)
In other words, to be anything at all is to be made with certain limitations; part of happiness is learning to accept those limitations.
We also hate to accept that the resources given to us have limitations. My bank account has a certain amount of money; when that amount of money goes away, I have nothing more to spend. I have 24 hours in a day to give; when those are filled, that's that.
The book of Ecclesiastes is in essence about learning to accept these limitations as part of the secret to a content life. But it also teaches us about a third kind of limitation: the limitation of created things to make us happy. Ecclesiastes seems like such a downer book for a reason - the author wants his readers to know that he's tried everything and found it ... limited. Money? He became the wealthiest king in his region, and it was vanity. Pleasure? He took wives and concubines, brought the best of the world to himself, and found nothing lasting. Knowledge? He hoarded knowledge like gold, and even it didn't satisfy on its own.
All those things are good; but they're limited, because they weren't made to satisfy us. As Augustine wrote, "Thou hast made us for Thyself, O Lord, and our hearts are restless until they rest in Thee" (Confessions). Finding satisfaction and true humanity includes learning that the good things of this life have limitations too; we're made to be satisfied only by the limitless love of God.