Mark 8:35

(ubi sup.) And this He says, because it may happen that a man may suffer and yet not follow Christ, that is, when he does not suffer for Christ's sake; for he follows Christ, who walks after Him, and conforms himself to His death, despising those principalities and powers under whose power, before the coming of Christ, he committed sin. Then there follows, For whosoever will save his life shall lose it; but whosoever shall lose his life for my sake and the Gospel's, the same shall save it. I give you these commands, as it were to spare you; for whosoever spares his son, brings him to destruction, but whosoever does not spare him, saves him. It is therefore right to be always prepared for death; for if in the battles of this world, he who is prepared for death fights better than others, though none can restore him to life after death, much more is this the case in spiritual battle, when so great a hope of resurrection is set before him, since he who gives up his soul unto death saves it.

And life is to be taken in this place for the present life, and not for the substance itself of the soul.

(in Marc. 2, 36) Or else He says this, because in time of persecution, our life is to be laid aside, but in time of peace, our earthly desires are to be broken, which He implies when He says, For what shall it profit a man, &c.