John Chrysostom

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. …

Catena Aurea by Aquinas ↗

Theophylact of Ohrid

Since the command to give oneself over to death would seem harsh and cruel, the Lord says that on the contrary it is most merciful; for whoever loses his soul, but for My sake — and not as a robber being executed or a suicide (for in that case the death would not be for My sake) — he, He says, will find his soul, while the one who thinks to save his soul will lose it, if during the time of torment he does not stand firm.

Commentary on Mark ↗

Augustine of Hippo

This precept by which we are enjoined to lose our life does not mean that a person should kill himself, which would be an unforgivable crime, but it does mean that one should kill that in oneself which is unduly attached to the earthly, which makes one take inordinate pleasure in this present life to the neglect of the life to come. …

Letter 243, To Laetus ↗

Gregory the Dialogist

For he that will save his life shall lose it; and he that shall lose his life for my sake and the gospel's shall save it. Thus it is said to the faithful: He that will save his life shall lose it; and he that shall lose his life for my sake shall save it. As if it were said to the farmer: If you keep your grain, you lose it; if you sow it, you renew it. For who does not know that when grain is cast as seed, it perishes from sight, it fails in the earth? But from where it rots in the dust, from there it springs up green in renewal. …

Forty Gospel Homilies, Homily 32 ↗

Bede

For whoever wants to save their soul will lose it. But whoever loses their soul for my sake and the gospel will save it. Thus it is said to the faithful: Whoever wants to save their soul will lose it. But whoever loses their soul for my sake and the gospel will save it. As if it were said to a farmer: If you save the grain, you lose it; if you sow it, you renew it. For who does not know that when grain is sown, it disappears from sight, perishing in the ground? But from where it rots in the dust, it springs forth in renewal. …

On the Gospel of Mark ↗