10 And now, by way of exhortation to follow in the path of His own passion, He adds, “He that loves his life shall lose it,” which may be understood in two ways: “He that loves shall lose,” that is, If you love, be ready to lose; if you would possess life in Christ, be not afraid of death for Christ. Or otherwise, “He that loves his life shall lose it.” Do not love for fear of losing; love it not here, lest you lose it in eternity. But what I have said last seems better to correspond with the meaning of the Gospel, for there follow the words, “And he that hates his life in this world shall keep it unto life eternal.”
So that when it is said in the previous clause, “He that loves,” there is to be understood in this world, he it is that shall lose it. “But he that hates,” that is, in this world, is he that shall keep it unto life eternal. Surely a profound and strange declaration as to the measure of a man's love for his own life that leads to its destruction, and of his hatred to it that secures its preservation! If in a sinful way you love it, then do you really hate it; if in a way accordant with what is good you have hated it, then have you really loved it.
Happy they who have so hated their life while keeping it, that their love shall not cause them to lose it. But beware of harboring the notion that you may court self-destruction by any such understanding of your duty to hate your life in this world. For on such grounds it is that certain wrong-minded and perverted people, who, with regard to themselves, are murderers of a specially cruel and impious character, commit themselves to the flames, suffocate themselves in water, dash themselves against a precipice, and perish.
This was no teaching of Christ's, who, on the other hand, met the devil's suggestion of a precipice with the answer, “Get behind me, Satan; for it is written, You shall not tempt the Lord your God.” To Peter also He said, signifying by what death he should glorify God, “When you were young, you girded yourself, and walked whither you would, but when you shall be old, another shall gird you, and carry you whither you would not;” — where He made it sufficiently plain that it is not by himself but by another that one must be slain who follows in the footsteps of Christ.
And so, when one's case has reached the crisis that this condition is placed before him, either that he must act contrary to the divine commandment or quit this life, and that a man is compelled to choose one or other of the two by the persecutor who is threatening him with death, in such circumstances let him prefer dying in the love of God to living under His anger, in such circumstances let him hate his life in this world that he may keep it unto life eternal.
Source: Tractates on the Gospel of John (New Advent)