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John Gill
Iron sharpeneth iron,.... A sword or knife made of iron is sharpened by it; so butchers sharpen their knives; so a man sharpeneth the countenance of his friend; by conversation with him; thus learned men sharpen one another's minds, and excite each other to learned studies; Christians sharpen one another's graces, or stir up each other to the exercise of them, and the gifts which are bestowed on...
Matthew Henry
1 Boast not thyself of to morrow; for thou knowest not what a day may bring forth. Here is, 1. A good caution against presuming upon time to come: Boast not thyself, no, not of to-morrow, much less of many days or years to come. This does not forbid preparing for to-morrow, but presuming upon to-morrow. We must not promise ourselves the continuance of our lives and comforts till to-morrow, but...
Charles H. Spurgeon
Iron sharpeneth iron; so a man sharpeneth the countenance of his friend. Hence the usefulness of Christian association, and hence also the evil of sinful company, for one sinner sharpens another to do mischief, just as one saint encourages another to righteousness.