Ecclesiastes 3:1

The scope of these verses is to show, 1. That we live in a world of changes, that the several events of time, and conditions of human life, are vastly different from one another, and yet occur promiscuously, and we are continually passing and repassing between them, as in the revolutions of every day and every year. In the wheel of nature (Jam 3:6) sometimes one spoke is uppermost and by and by the contrary; there is a constant ebbing and flowing, waxing and waning; from one extreme to the other does the fashion of this world change, ever did, and ever will. 2. That every change concerning us, with the time and season of it, is unalterably fixed and determined by a supreme power; and we must take things as they come, for it is not in our power to change what is appointed for us. And this comes in here as a reason why, when we are in prosperity, we should by easy, and yet not secure - not to be secure because we live in a world of changes and therefore have no reason to say, Tomorrow shall be as this day (the lowest valleys join to the highest mountains), and yet to be easy, and, as he had advised (Ecc 2:24), to enjoy the good of our labour, in a humble dependence upon God and his providence, neither lifted up with hopes, nor cast down with fears, but with evenness of mind expecting every event.

— Matthew Henry's Concise Commentary (public domain)