Proverbs 17:17

This intimates the strength of those bonds by which we are bound to each other and which we ought to be sensible of. 1. Friends must be constant to each other at all times. That is not true friendship which is not constant; it will be so if it be sincere, and actuated by a good principle. Those that are fanciful or selfish in their friendship will love no longer than their humour is pleased and their interest served, and therefore their affections turn with the wind and change with the weather. Swallow-friends, that fly to you in summer, but are gone in winter; such friends there is no loss of. But if the friendship be prudent, generous, and cordial, if I love my friend because he is wise, and virtuous, and good, as long as he continues so, though he fall into poverty and disgrace, still I shall love him. Christ is a friend that loves at all times (Joh 13:1) and we must so love him, Rom 8:35. 2. Relations must in a special manner be careful and tender of one another in affliction: A brother is born to succour a brother or sister in distress, to whom he is joined so closely by nature that he may the more sensibly feel from their burdens, and be the more strongly inclined and engaged, as it were by instinct, to help them.

— Matthew Henry's Concise Commentary (public domain)

To the second of these parallel clauses, there is an accession of meaning, that is, that a brother's love is specially seen in adversity.

— Jamieson-Fausset-Brown Commentary (public domain)