Charles H. Spurgeon on Psalms 27:4

One thing. The first thing, then, is David's choice, summarily described in the word, "one thing." So Christ confirmeth the prophet's word, while he called Mary's choice, "one thing." Lu 10:42. And that for these three reasons: First, because it is not a common but a chief good. If there be any good above it, it is not the chief good; and if there be any good equal unto it, it is not alone. Next, because it is the last end which we mind eternally to enjoy; if there be any end beyond it, it is not the last, but amidst, and a degree to it. All mids and ends are used for it, but it is sought for itself, and, therefore, must be but one. Thirdly, it is a centre whereunto all reasonable spirits draw. As all lines from a circle meet in the centre, so every one that seeketh happiness aright meeteth in the chief good, as the only thing which they intend, and, therefore, must be one. William Struther, in "True Happiness, or King David's Choice," 1633.