St. Francis de Sales on Friendship

Of Real Friendship
Of the Difference between True and False Friendship
Remedies against Evil Friendships

From Of Sadness and Sorrow:

The "sorrow of the world" disturbs the heart, plunges it into anxiety, stirs up unreasonable fears, disgusts it with prayer, overwhelms and stupefies the brain, deprives the soul of wisdom, judgment, resolution and courage, weakening all its powers; in a word, it is like a hard winter, blasting all the earth's beauty, and numbing all animal life; for it deprives the soul of sweetness and power in every faculty.