Translated by Ralph Anzarouth and an anonymous friend
Love of Friends and Comrades
The third category, which is also called love, fraternity, peace and friendship, involves pleasing each other [and treating each other] with truth and justice. It is pleasant, agreeable and precious for people to establish between themselves this love, based on fraternity and joining together. This love can result from one of three causes: the first is the gain and benefit one hopes to receive from his fellow man and this is what our Sages of blessed memory said (Ethics of the Fathers 5, 16): "Every love which is dependent upon something, when this thing vanishes love is extinguished with it". For this reason, it is an inferior and despicable kind of love. The second is providing mutual benefit in the same way as one hand washes the other; and for this reason it is called a worthy love. The third is when the one who loves seeks to benefit his friend in every way even if he himself will suffer damage or loss as a result. This is the perfect love which cannot be surpassed. The proof of this pure love is when one loves his friend with all his heart and will try to do everything he can think of to please him and refrain himself from doing anything which he deems unworthy or harmful to him. In fact, one acquires and keeps a friend with three things: by respecting him in his presence, by praising him when not in his presence and by rendering him service when needed. In addition, he will not show friendship in good times and turn away and distance himself in times of trouble. Talmud Bavli says (treatise Shabbat 32a): "It is well known that at the thresholds of wealthy shops abound friends and sympathizers; whereas at the thresholds of misery there are no friends or sympathizers". It is further known that the prime example of faithful friends is David and Jonathan, whose wonderful friendship in life and death is related in the Bible.
The wise man said that there are four things which improve with age: wine, fish1, oil and above all, friendship. Aristotle said that in the same way as a large tree needs a lot of attention, an important man needs many friends, since man can only be happy in this world in the company of friends and not when he is alone. And [our Sages] said (Talmud Bavli, treatise Taanit 23a): "Either friendship or death". Archita the philosopher said that if man were to ascend to the heavens and see the sun and its course in its extremities, the beauty of the moon and the stars in their positions in firmament and all the rest of the supernal renewals - and then descend [back to Earth] and see the marvels of the universe and all its contents - if he were alone and could not describe them to a company of friends he would have no pleasure and only great pain from them.
Our Sages of blessed memory said (Talmud Bavli, treatise Berachot, 17a): "A pearl of wisdom from Abaye was that man should always be wise in his awe of Hashem, he should give his replies gently, answer with warmth and spread peace among his brothers, his relations and all his fellow men, so that he will be loved in Heaven, desired in this world and accepted among all creatures".
Bear in mind that man changes and transforms himself according to his circle of friends. He should therefore acquire for himself a good friend. Seneca used to say: "Before you choose a friend, test him and try him, and after he passed the test love him with all your heart."
We found in Roman literature a tale concerning a king of Sicily2 who decreed upon a man called Pythias3 a sentence of death by beheading. Pythias asked from the king a reprieve of eight days in order to give him time to return to his home and put his affairs in order. The king replied by way of joke that his request would be granted if he could find somebody who would remain in prison in his place and would undertake to be beheaded in his place if he failed to return at the appointed time. Pythias sent for his friend Damon, who shared with him a very close mutual friendship and told him his predicament. Damon immediately went to the king and undertook to be beheaded if Pythias did not return by the end of the eight days. They imprisoned him until [the time fixed for] Pythias's return. As the deadline approached and Pythias had not come, everybody made fun of poor Damon for undertaking what he did. Damon, however, was not at all concerned since he fully trusted his loyal friend. And so it was that when the deadline was reached Pythias returned as he had promised. When the king saw such intense and faithful friendship, he pardoned Pythias since he did not want to separate by death two friends such as these.
Notes of the translators:
 We dare to assume that the author refers to live fish.
 This was Dionysius.
 Originally spelled 'Phintias'.
The whole book Tzemach Tzadik in Hebrew (printed in Rashi characters) can be downloaded in PDF format and read online at Hebrewbooks.org. Chapter 4 about love of friends is found at pages 12-14.